Hypoxic fractions of solid tumors: experimental techniques, methods of analysis, and a survey of existing data

Manipulation of oxygenation in a human tumour xenograft with BW12C or hydralazine: effects on responses to radiation and to the bioreductive cytotoxicity of misonidazole or RSU-1069

The influence of altered tumour oxygenation on the responses to radiation and/or bioreductive 2-nitroimidazole compounds was studied in a well differentiated, human, colon adenocarcinoma (MAWI), grown as a subcutaneous xenograft in nude mice. Tumour growth delays were measured after local, single 5-18 Gy doses of X-rays. BW12C, which inhibits dissociation of oxyhaemoglobin, produced radioprotection similar to that resulting from clamping off the tumour blood supply during irradiation. Hydralazine, a vasoactive agent, also appeared to give radioprotection. BW12C or misonidazole (MISO) alone had no measurable inhibitory effect on xenograft growth. Hydralazine or RSU-1069 slightly increased the time for tumours to reach 6 times their original volumes. When hydralazine was given 40 min after a dose of 800 mg/kg of MISO, without X-rays, growth delays in excess of 5 tumour volume doubling times resulted and fewer tumour cells were present in histological sections. Lower doses of MISO combined with hydralazine were ineffective. Other combinations of bioreductive cytotoxic agents and methods of manipulation of tumour blood flow/oxygenation induced slight and inconsistent growth delays. Hydralazine was injected after irradiation of tumours in mice previously treated with various doses of MISO in an attempt to exploit the bioreductive cytotoxic potential of MISO in conjunction with its radiosensitizing properties; however, tumour growth delays were similar with or without hydralazine after irradiation. Thus, post-irradiation restriction of tumour blood flow appears to be an ineffective therapeutic strategy in this human xenograft tumour model.

Radiolabelled fluoromisonidazole as an imaging agent for tumor hypoxia

Fluoromisonidazole labeled with H-3 or F-18 has been tested as a quantitative probe for hypoxic cells in vitro and in rodent and spontaneous dog tumors in vivo. In V-79, EMT-6(UW), RIF-1, and canine osteosarcoma cells in vitro, the binding of 50 microM [H-3]Fluoromisonidazole was 50% inhibited by 1000-2000 ppm O2, relative to binding under anoxic conditions. After a 3 hr incubation with labeled drug, the anoxic/oxic binding ratios ranged from 12 to 27 for the four cell types. Retention of [H-3]fluoromisonidazole 4 hr after injection was greater in large KHT tumors (400-600 mm3) with an estimated hypoxic fraction greater than 30%, than in smaller tumors (50-200 mm3) with an estimated hypoxic fraction of 7-12%. RIF-1 tumors, with an estimated hypoxic fraction of 1.5%, retained the least label, with tumor: blood ratios ranging from 1.7 to 1.9. Spontaneous dog osteosarcomas were imaged with a time of flight positron emission tomograph for up to 5 hr following injection of [F-18] fluoromisonidazole. Analysis of regions of interest in images allowed creation of dynamic tissue time activity curves and calculation of tissue uptake in cpm/gram. These values were compared to radioactivity in plasma. In all cases, retention in some tumor regions exceeded that in plasma and in normal tissue, such as muscle or brain, by 3 to 5 hr post injection. Uptake of fluoromisonidazole in tumors was heterogeneous, with ratios of maximum to minimum uptake as high as 4 in different regions of interest in the same tumor. Tumor:plasma values ranged from 0.28 to 2.02. The oxygen dependency of fluoromisonidazole retention was similar in a variety of cell types and was 50% inhibited by O2 levels in the transition between full radiobiological hypoxia and partial sensitization. The quantitative regional imaging of [F-18] fluoromisonidazole in spontaneous canine tumors at varying times post-injection lays the basis for imaging and modeling of oxygen-dependent drug retention in different regions of human neoplasms.

Conversion of xanthine dehydrogenase to xanthine oxidase as a possible marker for hypoxia in tumours and normal tissues

The enzyme activities of endogenous xanthine dehydrogenase (XDH) and xanthine oxidase (XO) have been measured in 10 different types of mouse tumour and seven normal tissues. The conversion of XDH to XO has been observed in two tumour types upon the prolonged clamping off of the blood supply to the tumours. It is proposed that a similar conversion might also occur naturally in chronically hypoxic cells and that the ratio of the XO activity to the combined XO + XDH activities (%XO activity) could well serve as a marker for tissue hypoxia. A qualitative relationship exists between the %XO activity and literature values of the hypoxic fraction for some tumours measured by radiobiological assays. The influence of tumour size (about 0.2-1.8 g) on %XO activity is presented for all 10 tumours as well as %XO activity determinations for four of the normal tissues.

Hypoxia and reoxygenation in human melanoma xenografts

Tumor hypoxia and reoxygenation pattern following single dose (10.0 Gy) and fractionated (7 fractions of 2.0 Gy, 1 fraction per day) irradiation were studied in five human melanoma xenograft lines using the paired survival curve method. The hypoxic fractions differed significantly among the melanoma lines; they were found to be 6 +/- 3% (E.E.), 22 +/- 8% (E.F.), 31 +/- 11% (G.E.), 45 +/- 17% (M.F.), and 15 +/- 5% (V.N.). There were no clear correlations between hypoxic fraction and tumor volume-doubling time or vascular density, suggesting that intrinsic cellular characteristics, for example, rate of oxygen consumption and ability to retain clonogenicity under hypoxic stress, also may play an important role for the magnitude of the hypoxic fractions in the melanomas. Reoxygenation was rapid and extensive in all melanoma lines; 12-24 hr after the single dose irradiation or the last fraction of the fractionated irradiation, the hypoxic fractions were similar to those in untreated tumors and stayed at that level up to at least 10 days after irradiation. The hypoxic fractions 1-10 days after irradiation tended to be higher after fractionated than after single dose irradiation, but the differences were not statistically significant. There was a positive correlation between the hypoxic fractions in untreated tumors and the hypoxic fractions after irradiation and reoxygenation, suggesting that it may be possible to predict radiation resistance caused by hypoxia from the hypoxic fractions in tumors before start of radiation therapy. However, hypoxia is probably not a major cause of failure in the radiation therapy of malignant melanoma.

The effect of arterial oxygen content on the results of radiation therapy for epidermoid bronchogenic carcinoma

Anemia is believed to be an important prognostic factor in treating cancer patients by radiation therapy. One possible explanation for this is tumor oxygenation. With respect to tumor oxygenation, the arterial oxygen content (CaO2) may be of more importance than the hemoglobin (Hb) level. This study shows the relationship between the CaO2 and tumor response to radiation therapy. Forty-two patients with epidermoid bronchogenic carcinoma, treated by irradiation alone between April 1982 and March 1986, were reviewed. Regression of the tumor after radiation therapy was calculated as a percent change in the tumor area. Arterial oxygen partial pressure (PaO2), Hb level, and percent oxygen saturation of Hb in arterial blood (SaO2) were measured within 2 weeks of commencement of radiation therapy and the CaO2 was calculated. The rank correlation coefficients between the maximum percent regression of the tumor and the PaO2, the Hb level and the CaO2 were 0.36, 0.34, and 0.46, respectively. Statistical analyses of the data indicate that the group of patients with CaO2 over 14.5 ml/dl exhibited greater tumor regression and longer survival periods than the group of patients with CaO2 below 14.5 ml/dl. Similarly patients with PaO2 over 90 mmHg or Hb level over 11 g/dl, exhibited significantly greater tumor regression and longer survival periods than those with PaO2 below 90 mmHg and Hb level below 11 g/dl. There were no significant differences in the length of survival periods with respect solely to the Hb level or the PaO2. It was concluded that the CaO2 is more important than the Hb level in determining tumor response to radiation therapy. This is considered as important indirect evidence of the existence of hypoxic fractions of cells in human tumors.

Effect of the manipulation of oxyhemoglobin status by BW12C on tumor thermosensitivity and on blood flow in tumor and normal tissues in mice

The effect of 70 mgkg-1 BW12C 30 min before heating on the thermosensitivity of RIF-1 leg tumors was studied. This schedule is known to increase the hypoxic fraction by a factor of 5. Heating, using a combined radio-frequency and saline bath technique, was for 30 min at 43, 43.5, and 44 degrees C and response was assayed by clonogenic cell survival immediately and 24 hr after treatment. BW12C did not alter RIF-1 thermosensitivity. The effects of heat up to 44 degrees C on the oxygen saturation curves of normal and BW12C-modified blood were compared and P50s were shown to rise from 36 to 52 mm Hg and 6.5 to 8.0 mm Hg respectively, showing the latter to be relatively resistant to right-shifting by heat. 86Rb extraction studies on BW12C-treated unheated animals showed that blood flow in leg and flank tumours 60 min after BW12C was reduced to 64% and 34% of control values respectively, indicating a further mechanism for induction of tumour hypoxia by BW12C. Blood flow in leg muscle, liver, and spleen was unchanged but in kidney and lung was increased to 127% and 119% of control respectively 60 min after BW12C.

The effect of glutathione (GSH) depletion in vivo by buthionine sulfoximine (BSO) on the radiosensitization of SR 2508

The effect of glutathione depletion (GSH) on the efficacy of SR 2508 was evaluated in two murine tumor models with single large doses of radiation or with low doses administered in an accelerated fractionated schedule. To deplete tumor GSH, buthionine sulfoximine (BSO) was administered in the animals drinking water (10 mM) following two i.p. injections of 450 mg/kg. This treatment decreased RIF and MCA tumor GSH concentrations by 95% and 80%, respectively. Mice (C3H/Sed) received BSO for 48-72 hr before the first dose of radiation, and were maintained on BSO drinking water for the duration of the fractionated course of therapy. SR 2508 (200-1000 mg/kg) was injected 45 min prior to each fraction of radiation. Radiation was administered as a single dose of 15 Gy or 20 Gy, for RIF and MCA tumors, respectively. Alternatively, animals received a fractionated course of radiotherapy which consisted of 2.5 Gy/fraction for the RIF, and 3 Gy/fraction for the MCA tumors, b.i.d. for five days (total of 10 fractions). Tumor response with and without BSO, and with and without SR 2508, was determined by regrowth delay. BSO pretreatment increased the efficacy of SR 2508 with single dose radiation in the MCA but not RIF tumor. SR 2508 administered with fractionated radiation produced lower enhancement ratios (SER) than with a single radiation dose. However, BSO significantly enhanced the efficacy of SR 2508 with fractionated radiation. BSO increased the maximum SER for SR 2508 (3 mM/fraction) from 1.2 to 1.4 in the RIF tumor, and from 1.4 to 1.8 in the MCA tumor. BSO also increased the toxicity of SR 2508 by a factor of 2. However, the ability of BSO to increase the efficacy of low doses of sensitizer at clinically relevant doses of radiation suggests that this combined modifier treatment may be of clinical benefit.

Micropharmacokinetics of chemical modifiers

Classical pharmacokinetic analysis of plasma, urine and tissue specimens continues to be of major value to the rational development of chemical modifiers of cancer treatment. However, in addition, increasingly sophisticated analytical techniques are becoming available which allow the pathways of microdistribution and micrometabolism of drugs to be traced down to the cellular and molecular level. New developments described here include flow cytometry, magnetic resonance spectroscopy, and molecular enzymology. These are predicted to have a major impact on the optimization of chemical modification.

Demonstration of tumor-selective retention of fluorinated nitroimidazole probes by 19F magnetic resonance spectroscopy in vivo

We have evaluated two fluorinated misonidazole analogues, Ro 07-0741 and CCI-103F, as potential probes for the non-invasive identification of hypoxic tumor cells by 19F magnetic resonance spectroscopy (MRS) in vivo. The equipment used was a 1.9 T Oxford Research Systems TMR-32 spectrometer, fitted with a 15 mm diameter surface coil. Signal was readily detectable, with similar intensity from EMT6 tumor, liver, and brain at early times (1-2 hr) after i.v. injection in BALB/c mice, indicative of an initial uniform biodistribution of parent probes. At later times (5-10 hr) there was a progressive reduction in signal intensity from brain and liver, but tumor levels remained constant or declined more slowly. This is illustrated by tumor/brain ratios at 6-7 hr of 2.9 (Ro 07-0741) and 4.2 (CCI-103F). In 4/5 mice analyzed at 20-24 hr after Ro 07-0741, and 1/2 following CCI-103F, tumor signal remained detectable. This occurred in the absence of parent probe as measured by HPLC, suggesting the involvement of a product of nitroreductive bioactivation. Studies with KHT and RIF-1 tumors in C3H/He mice showed a similar trend but retention in RIF-1 was less dramatic, and this was consistent with the known hypoxic fractions and comparative in vivo nitroreductase activities. These promising results support the continuing development of 19F nitroimidazole probes for non-invasive identification of hypoxic cells in vivo.

Selective fractionation of hypoxic B16 melanoma cells by density gradient centrifugation

Cells of the mouse B16 melanoma growing in monolayer culture and as tumors were fractionated by isopycnic density centrifugation in a linear-density (1.02-1.20 g/ml) metrizamide gradient. Cultured cells concentrated into one or two distinct bands, with densities of 1.02-1.04 g/ml and 1.06-1.10 g/ml, depending on growth conditions. Cells subjected to extreme hypoxia (less than 0.02% O2) banded predominantly at the lower density, and normally-oxygenated cells banded at the higher density. Fractionated tumor cells concentrated at both densities. Compared with cells at the higher density, lower-density cells incorporated more of the hypoxic cell radiosensitizer [14C]misonidazole and less [3H]thymidine in vivo, were less clonogenic but more resistant to X-irradiation in situ, and labeled to a lesser extent with intravenously-delivered Hoechst 33342 fluorochrome, a marker for cells proximal to tumor blood vessels. Lower-density tumor cells were, therefore, enriched in non-proliferating radioresistant hypoxic cells from tumor regions remote from blood vessels.

Biological effectiveness of fast neutrons on a murine osteosarcoma

The effect of fast neutrons and gamma rays on a murine osteosarcoma was studied. The NROS tumor, a radiation-induced osteosarcoma in a C3H mouse, was transplanted into the right hind legs of syngeneic female mice and locally irradiated with single or four daily doses of either fast neutrons or gamma rays. The NROS contained 13-30% hypoxic cells. It took approximately 7 days for the NROS tumor to show apparent reoxygenation following gamma ray irradiations. Two assays were used to determine the neutrons' relative biological effectiveness (RBE) to gamma rays: tumor growth delay time and tumor control dose. The largest RBE of 4.5 was obtained at the smallest dose of neutrons examined, followed by a gradual decrease down to 2.3. The tumor growth delay assay indicated that the RBE values of 2.6-3.1 after single doses of fast neutrons increased to 3.1-4.5 after four daily fractions. The 50% tumor control doses were 78.5 Gy and 33.0 Gy after single doses of gamma rays and fast neutrons, resulting in an RBE of 2.3. Fractionated doses increased the RBE to 2.6. Mitotic cells disappeared shortly after irradiation but reappeared 7 days after irradiation.

Misonidazole binding in SCCVII tumors in relation to the tumor blood supply

Misonidazole (MISO) binding was examined in murine squamous cell carcinomas as a function of distance from the tumor blood supply. C3H mice bearing subcutaneous SCCVII tumors were injected intraperitoneally with 3H-MISO followed at various times later by intravenous injection or infusion of the fluorescent perfusion probe, Hoechst 33342. Tumors were then excised, and single cell suspensions prepared for fluorescence activated cell sorting. Cells sorted on the basis of Hoechst 33342 fluorescence were examined for 3H-MISO content by liquid scintillation counting or autoradiography. MISO content in the 10% of cells most distant from the blood supply was as much as 8 times greater than the amount in the 10% of cells closest to the blood supply. The largest differentials in MISO content between dimly and brightly staining regions were obtained if (a) tumors greater than about 300 mg were used, (b) at least 3 hr were allowed for MISO metabolism and binding prior to analysis, and (c) cell sorting was performed on the basis of concentration of Hoechst (correcting for cell size) rather than on the basis of total fluorescence intensity per cell. As tumors enlarged, MISO content increased even in the cells closest to the blood supply suggesting a decrease in net tumor oxygenation perhaps caused by intermittent hypoxia.

Growth and radiation sensitivity of the MLS human ovarian carcinoma cell line grown as multicellular spheroids and xenografted tumours

The growth characteristics and the radiation sensitivity of multicellular spheroids of the MLS human ovarian carcinoma cell line grown in spinner culture in atmospheres of 5% CO2 in air or 5% CO2, 5% O2 and 90% N2 were studied and compared to that of MLS xenografted tumours. The spheroids grew exponentially with a volume-doubling time of approximately 24 h up to a diameter of approximately 580 microns and then the growth rate tapered off, more for spheroids grown at the low than at the high oxygen tension. Thirty days after initiation, the spheroid diameters were approximately 1,500 microns at the low and 2,100 microns at the high oxygen tension. The tumour volume-doubling times were approximately 8 days (V less than 200 mm3) and 17 days (V = 1,000-4,000 mm3). The histological appearance of the spheroids and the tumours was remarkably similar; both developed large central necrosis and both were composed of epithelial cells and showed pseudoglandular structures with lumen. The spheroids were slightly less differentiated than the tumours. The intrinsic, cellular radiation sensitivity was independent of whether the cells were grown in vitro as spheroids or in vivo as tumours, as revealed by irradiating single cells from dissociated spheroids and tumours under aerobic conditions and intact spheroids and tumours under hypoxic conditions. Studies of 1,600 microns spheroids grown in 5% CO2 in air showed that the intrinsic radiation sensitivity of the chronically hypoxic cells was the same as that of acutely hypoxic cells. The fraction of radiobiologically hypoxic cells under these conditions was approximately 15% and similar to those of 9% (V = 200 mm3) and 28% (V = 2,000 mm3) found for the tumours. Spheroids with diameter of 1,200 microns did not show survival curves parallel to those for acutely hypoxic cells, i.e. they did not contain a measurable fraction of clonogenic cells at complete radiobiological hypoxia. The final portion of their survival curves represented partially hypoxic cells; the OERs were 1.6 and 1.3 for spheroids grown at the high and the low oxygen tension, respectively. The considerable similarity between the spheroids and the tumours suggests that MLS spheroids constitute a valuable in vitro model for studies of human tumour radiation biology and related physiological processes. MLS spheroids may be particularly useful in studies of therapeutic consequences of partial radiobiological hypoxia since complete hypoxia and different levels of partial hypoxia can be studied separately by varying spheroid size and the oxygen tension in the culture medium.

Tumor radiation responses and tumor oxygenation in aging mice

EMT6 mouse mammary tumors transplanted into aging mice are less sensitive to radiation than are tumors growing in young adult animals. We hypothesized previously that this reflected a greater proportion of radiation resistant, hypoxic cells in the tumors of aging animals. The experiments reported here compare the radiation dose-response curves defining the survivals of tumor cells in aging mice and in young adult mice. Cell survival curves were assessed in normal air-breathing mice and in mice which had been asphyxiated with N2 to produce uniform hypoxia throughout the tumors. Analyses of these survival curves revealed that 41% of the viable malignant cells were severely hypoxic in tumors in aging mice, while only 19% of the tumor cells in young adult animals were radiobiologically hypoxic. This did not appear to reflect anemia in the old animals, as the hematocrits of young and aging tumor-bearing animals were similar. Treatment of aging animals with a perfluorochemical emulsion plus carbogen (95% O2/5% CO2) increased the radiation response of the tumors, apparently by improving tumor oxygenation and thereby decreasing the number of severely hypoxic, radiation resistant cells in the tumors.

No-carrier-added synthesis of 3-[18F]fluoro-1-(2-nitro-1-imidazolyl)-2-propanol. A potential PET agent for detecting hypoxic but viable tissues

Four different approaches towards the synthesis of [18F]FMISO have been studied. The first approach was based on the reaction of epoxide 4 and [18F]fluoride. Both specific activity and radiochemical yield (less than 1%) for [18F]FMISO were low. Two new approaches, starting with compounds 8 and 9, have failed to give [18F]FMISO. The fourth approach, based on the reaction of [18F]epifluorohydrin 10, prepared from Tosylate 13 and [18F]KF/Kryptofix 222, has provided a reliable, no-carrier added synthesis of [18F]FMISO. The product was obtained in a radiochemical yield of 7-12% at end-of-synthesis (based on [18F]fluoride) with a specific activity of greater than 400 Ci/mmol and a synthesis time of 1.5 h. Preliminary PET studies suggest that [18F]FMISO may be a promising tracer for delineation of ischemic but viable myocardium.

Phosphorus-31 magnetic resonance spectroscopy and blood perfusion of the RIF-1 tumor following X-irradiation

Phosphorus-31 magnetic resonance spectra were obtained from the RIF-1 tumor in C3H mice before and up to 2 days after various doses of X rays. Parallel studies were performed to measure relative changes in tumor blood perfusion using [14C]iodo-antipyrine and changes in % tumor necrosis using Chalkley's method. Tumor ratios of phosphocreatine to inorganic phosphate (PCr/Pi) and nucleotide triphosphates to inorganic phosphate (NTP/Pi) as well as pH as measured by 31P-MRS increased significantly at most time points after irradiation with doses of 5, 10, and 20 Gy. Tumor blood perfusion was found to significantly improve after a dose of 20 Gy but not after a dose of 2 Gy. Percent tumor necrosis increased to about 3 times its control level at 1 day after a dose of 20 Gy and then declined to about twice its control value at 2 days. The magnitude of the changes in the 31P-MRS parameters makes it unlikely that any of them are entirely due to radiation-induced changes in the radiobiologically hypoxic fraction of these tumors. Changes in the necrotic fraction did not appear to influence the tumor spectra. However, the observed improvement in tumor blood perfusion may have resulted in an increase in oxidative phosphorylation of the whole tumor population as well as a clearance of inorganic phosphate and acid metabolites, so that 31P-MRS changes may indirectly reflect changes in tumor blood perfusion.

Effect of cancer chemotherapy on the hypoxic fraction of a solid tumor measured using a local tumor control assay

The effect of mitomycin C (MMC), adriamycin (ADM), cyclophosphamide (CTX), cisplatinum (cis-DDP) and bleomycin (BLM) on the aerobic and hypoxic cells of a C3H mammary carcinoma in CDF1 mice was investigated using the tumor control assay. Hypoxic fractions (HF) were calculated by an indirect technique using the horizontal displacement of the TCD50 value from the dose-response curves of tumors irradiated under normal or clamped conditions. The HF and absolute number of tumor cells following a combined treatment was compared to that obtained with radiation alone. MMC, ADM and CTX had a significant enhancing effect on the unclamped TCD50. All three drugs caused a marked reduction in the proportion of hypoxic cells, decreasing the HF from 5.4% to about 1% of the total cell number. The surviving proportion of hypoxic cells were 11.1, 8.9 and 6.5% respectively. Killing of aerobic cells was also observed but the effect was less than that seen on the hypoxic cells, with the survival only being reduced to between 38 and 68% of the total number of aerobic cells. In contrast, cis-DDP and BLM were shown to produce major cell killing in the aerobic compartment but actually showed no cytotoxicity towards hypoxic cells. This would explain the lack of radiation enhancement observed for these two drugs. We conclude that the ability of adjuvant drugs to improve radiation response is dependent on the hypoxic cell killing by the drugs.

Effect of Fluosol-DA on the response of intracranial 9L tumors to X rays and BCNU

Treatment with a perfluorochemical emulsion combined with breathing a 95% or 100% oxygen atmosphere has been shown to be an effective adjuvant to radiation therapy in several animal tumor systems. Similarly, the addition of treatment with a perfluorochemical emulsion combined with breathing a high oxygen atmosphere has been shown to improve the response of several animal tumor systems to treatment with BCNU. We now report results of the use of the perfluorochemical emulsion, Fluosol-DA, and carbogen breathing with single dose radiation treatment, BCNU and combined drug and radiation treatment in intracranially implanted 9L gliosarcoma. The median enhancement in life span produced by Fluosol-DA and carbogen breathing in addition to radiation was 2 days at 10 Gy and 6 days at 20 Gy compared to radiation treatment alone. In the group receiving 20 Gy with Fluosol-DA and carbogen breathing, 2 of 20 lived 120 days. Treatment with a single intraperitoneal injection of BCNU (10 mg/kg) on day 7 post tumor cell implantation produced an increase in life span of 2 days compared to untreated control animals. The combination of drug treatment with Fluosol-DA and carbogen breathing produced an increase in life span of 26 days, which was significantly different from BCNU treatment with air breathing (p less than 0.001). Finally, when BCNU and Fluosol-DA and carbogen were combined with radiation treatment (20 Gy), an increase in life span of nearly 85 days compared to untreated controls was produced, with 47% (9 or 19) surviving 120 days. These results suggest that this combination might be effective in the treatment of malignant brain tumors.

Preclinical studies of a perfluorochemical emulsion as an adjunct to radiotherapy

Tumor growth and tumor cell survival endpoints were used to examine the effects of a perfluorochemical emulsion, Fluosol-DA, 20%, and carbogen (95% O2/5% CO2) on EMT6 mouse mammary tumors in BALB/c mice. These studies defined the effects of the Fluosol dose on the hematocrit and fluorocrit of the mice and on the radiation response of the tumors. The effect of varying the duration of carbogen breathing before irradiation was examined; times of 5-60 min gave similar enhancements of tumor radiosensitivity. Potentiating effects were not observed when the tumors were irradiated 1-3 days after Fluosol injection, probably reflecting the observed clearance of the perfluorochemicals from the circulating blood. Fluosol injected 30 min-2 days before irradiation did not alter the radiation response of tumors in air-breathing or N2-asphyxiated mice. Together, these studies provided additional support for the hypothesis that the potentiation of tumor radiation response observed after treatment with Fluosol plus carbogen results from changes in O2 delivery to the hypoxic tumor cells by oxygenated perfluorochemical particles. This confirms the conclusion drawn on the basis of the observed changes in the tumor cell survival curve. Studies of tumor cell survival, tumor cell yield, tumor growth, and artificial lung metastasis formation revealed no effects of Fluosol treatment (without irradiation) on tumor progression or metastasis. Studies examining the effects of Fluosol plus carbogen on the growth of tumors irradiated with 5 Gy showed that the changes in tumor radiosensitivity observed using cell survival endpoints also occurred in tumors left in situ after irradiation with a radiation dose similar to those used in some clinical trials.

31P NMR spectroscopy in vivo of two murine tumor lines with widely different fractions of radiobiologically hypoxic cells

Energy and lipid metabolism as well as tumor pH in two murine tumor lines, the KHT and RIF-1 sarcomas, were studied using 31P NMR spectroscopy. Possible relationships between spectral parameters on the one hand and volume fraction of necrosis and fraction of radiobiologically hypoxic cells on the other were investigated. For both tumor lines the PCr and NTP beta resonances decreased and the Pi resonance increased significantly with increasing tumor volume in the volume range 100-4000 mm3. This decrease in bioenergetic status was accompanied by a decrease in tumor pH from about 7.2 to about 6.8. The NTP beta resonance and the tumor pH tended to be somewhat higher and the Pi resonance somewhat lower for the KHT than for the RIF-1 tumors. Linear relationships were found between tumor pH and Pi or (PCr + NTP beta)/Pi for both tumor lines (P much less than 0.05). The PME resonance increased slightly and the PDE resonance decreased slightly during tumor growth and were not significantly different for the KHT and the RIF-1 tumors. The volume fraction of necrosis was about 5 per cent in both lines at a tumor volume of 100 mm3 and increased to about 30 per cent (KHT) and 50 per cent (RIF-1) at a tumor volume of 4000 mm3. The fraction of radiobiologically hypoxic cells was found to increase from 12 to 23 per cent for the KHT line and from 0.9 to 1.7 per cent for the RIF-1 line when tumor volume was increased from about 200 to about 2000 mm3. The volume-dependence of the 31P NMR spectral parameters indicated increased nutritional deprivation and development of hypoxia and necrosis during tumor growth, and was thus qualitatively in good agreement with the changes observed in necrotic and hypoxic fraction. However, quantitative relationships between any spectral parameter and necrotic or hypoxic fraction across tumor lines were not found, implying that other physiological parameters and/or cellular characteristics may contribute significantly to a 31P NMR tumor spectrum. Consequently, 31P NMR spectra of untreated tumors have to be supplemented with other tumor data, e.g. rate of oxygen consumption, cell survival time under hypoxic stress and/or fraction of metabolically active, non-clonogenic hypoxic cells, to be useful in quantitative determination of tumor hypoxia and hence prediction of tumor radioresistance caused by hypoxia.

Additivity of radiosensitization by the combination of SR 2508 (etanidazole) and Ro 03-8799 (pimonidazole) in a murine tumor system

The nitroimidazole radiosensitizers SR 2508 and Ro 03-8799 have different dose-limiting toxicities in man and hence can be used in combination. Such therapy will be beneficial only if their radiosensitizing properties are additive, which this study sought to determine using clinically relevant radiosensitizer concentrations in the EMT6 tumor in the flanks of BALB/c mice. 240 mg/kg of each drug gave tumor concentrations (+/- 2 se) 55 min after i.v. administration of the combination of 50.4 +/- 10.6 micrograms/g (236 +/- 50 nmol/g) for SR 2508 and 39.7 +/- 9.0 micrograms/g (137 +/- 31 nmol/g) for Ro 03-8799. The radiosensitization by both agents administered both singly and in combination at 240 mg/kg and singly at 480 mg/kg was measured, giving sensitizers 30 min before 20 Gy of 250 kV X rays. Tumor response was assayed by clonogenic cell survival. SER values (with 95% confidence limits) were 1.28 (1.20-1.37) for 240 mg/kg SR 2508, 1.20 (1.10-1.30) for 240 mg/kg Ro 03-8799, 1.46 (1.33-1.59) for 240 mg/kg of both drugs in combination, 1.46 (1.38-1.55) for 480 mg/kg SR 2508 and 1.46 (1.31-1.62) for 480 mg/kg Ro 03-8799. These data confirm the additivity of radiosensitization by the two drugs administered in combination.

Characterization of radiation resistant hypoxic cell subpopulations in KHT sarcomas. (II). Cell sorting

Hypoxic cells in KHT sarcomas were characterized using fluorescence activated cell sorting based on the diffusion properties of the fluorochrome Hoechst 33342. Tumour-bearing female C3H/HeJ mice were injected i.v. with 10 micrograms g-1 Hoechst 33342 and the cells derived from the tumours sorted on the basis of their staining intensities. For each sorted fraction the DNA histogram was evaluated using FCM analysis. The results indicated that the bright and dim cells were not equally distributed about the cell cycle. For example, a greater proportion of S phase cells were in the bright subpopulations whereas the dim subpopulations contained an increased proportion of cells in G1. When the tumours were irradiated with a single dose of radiation prior to cell sorting, the dim cells survived preferentially. Dose response curves for the 20% most dim and 20% most bright cells, sorted on the basis of fluorescence intensity, then were determined. The survival curves of the dim and bright cells were found to have slopes similar to those of KHT cells irradiated in situ in dead animals or in vitro under fully oxic conditions, respectively. In addition, when KHT sarcoma-bearing mice were given a 2.5 mmol kg-1 dose of misonidazole (MISO) prior to irradiation and cell sorting, the dim subpopulation was sensitized whereas the bright subpopulation was not. These findings suggest that (i) compared to well-oxygenated areas, hypoxic regions of KHT tumours contain a smaller percentage of cells actively proliferating and (ii) Hoechst 33342 sorting may allow the detailed in situ evaluation of agents acting directly against hypoxic cells in solid tumours.

Variations in the spectrum of lesions produced in the DNA of cells from mouse tissues after exposure to gamma-rays in air-breathing or in artificially anoxic animals

Gamma-ray-induced DNA-protein crosslinks (dpc) are preferentially induced in cultured cells irradiated at very low oxygen tensions (Meyn et al. 1987). Since some cells within mouse tumors may be radiobiologically hypoxic, dpc may also be induced in such cells after irradiation in vivo. To examine this possibility, mice bearing either an FSa or NFSa fibrosarcoma in their hind legs were whole-body irradiated either while breathing atmospheric oxygen or 15 min after cervical dislocation, which induces uniform anoxia. DNA single-strand breaks (ssb) and dpc were then assayed both in tumors and normal tissues by alkaline elution. The level of dpc was inferred from the observed increase in ssb yield after digestion of the cell lysates with proteinase K. In addition, cell suspensions were irradiated in vitro, on ice, exposed to atmospheric oxygen tensions. Few dpc were detected in the DNA from tumor cells irradiated in vitro; however, in cells from both FSa and NFSa tumors irradiated in situ there was a significant level of protein-concealed ssb, and thus of dpc. These data are most likely the result of the relative hypoxia of a proportion of cells from both the FSa and NFSa tumor in the air-breathing animals. Induction of dpc was further enhanced in the DNA from tumor cells irradiated under anoxic conditions. A significant level of dpc was also observed in jejunal and spleen cells irradiated in vivo; however, since a significant level of protein-concealed breaks was also observed in cells irradiated in vitro, oxygenation appears not to be the only parameter capable of modifying the proportion of protein-concealed ssb, and the effects of proteinase K on the DNA elution rate for normal mouse tissues may be complex.

Estimation of tumor oxygenation and metabolic rate using 31P MRS: correlation of longitudinal relaxation with tumor growth rate and DNA synthesis

31P MRS longitudinal relaxation times (T1) were determined for C3H murine fibrosarcomas (FSaII), and mammary carcinomas (MCaIV). Tumors were implanted in the foot dorsum, and were 100-300 mm3 in volume. T1s were repeated after the animal was allowed to breathe 100% oxygen for 30 min and then again 36-48 hr following 30 Gy. The spectrum were obtained using an 8.5 T spectrometer with a 8 cm bore and a 1.4 cm single turn antenna coil. The 31P relaxation times for untreated tumors in air breathing animals were: 3.78 sec for phosphomonoesters, 4.37 sec for inorganic phosphate (Pi), 2.73 sec for phosphocreatine, 1.37 sec for gamma ATP, 1.14 sec for alpha ATP, and 1.18 sec for beta ATP. The Pi T1s were 4.37 and 4.70 sec in control and irradiated tumors in air breathing animals. Respiration of oxygen for 30 min reduced the T1s to 3.02 and 2.62 sec in control and irradiated tumors respectively. The Pi T1 of an anoxic tumor, determined on an in situ tumor 60 min after death was 5.93 sec. The oxygen breathing induced decrease in the T1 of Pi is unlikely to have been caused by the paramagnetic properties of oxygen alone, and suggests a component of increased magnetization transfer secondary to the ATPase reaction. Oxygen breathing following 30 Gy, resulted in a decreased growth time (800 mm3 endpoint) and an increased proportion of cells in S-phase. These results support the hypothesis that the decrease in Pi T1 measured with oxygen breathing is a measure of tumor oxygen tension and metabolic rate, and suggests that T1 measurement may indirectly predict tumor growth rate and DNA synthesis.

Applicability of animal tumor data to cancer therapy in humans

The problem of applying experimental tumor studies to clinical cancer therapy is a complex one. The radiotherapy literature contains many examples of premature efforts to apply laboratory observations to the clinic, and many examples of failures to adequately consider animal tumor observations in the design of clinical studies. This review covers three areas: tumor hypoxia, where clinical trials based on animal tumor data have been conducted with radiosensitizers, hyperbaric oxygen, and systemic oxygen carriers; dose fractionation, where current trials of hyperfractionation are based in part on animal tumor studies; and chemo-radiotherapy, where clinical trials are only beginning to exploit concepts developed in animal tumor systems. The use of animal tumor systems extends past the screening of new agents. Animal tumor models can be used in biological, physiological, and pharmacological studies to elucidate the biological factors influencing the efficacy of therapeutic agents. Tumor studies can be combined with studies of normal tissues to predict the toxicities to be anticipated in clinical trials, and to assess the potential for therapeutic gain. Animal studies can also provide data which are useful in designing optimal clinical trials of new agents and maximizing the potential for successful clinical application of new approaches. In general, it is not possible to apply specific laboratory data directly to man. To translate, rather than transpose, information from the laboratory to the clinic, the model studies must be directed at evaluating principles, rather than merely quantifying results. Only through studies of mechanisms, by designing experiments to test or refute a hypothesis, will it be possible to apply model studies to man.

Adenosine-5'-triphosphate levels in experimental CaNT and Fib/t tumours of varying volume and degree of hypoxia

The variation of adenosine-5'-triphosphate (ATP) content per unit mass of tumour, versus tumour volume was measured in vivo under normoxic conditions, using CaNT and Fib/t murine tumours grown in CBA and WHT mice respectively. A monotonically decreasing relation was found. Artificially induced tumour hypoxia resulting from 15 min of clamping was accompanied by reduced ATP levels.

The dynamics of tumor cords in an irradiated mouse mammary carcinoma with a large hypoxic cell component

The tumor cord model represents a histologically based framework for interpretation of radiobiological phenomena, particularly the resistance to radiation conferred by absence of oxygen. For the mammary carcinoma T50/80 grown in B6D2F1 male mice, average oxygenation was poor, based on tumor growth delay after irradiation. There was no improvement in radiobiological oxygenation for several days after a high dose of radiation. This was consistent with events in the cords of the tumor, where although up to 20% of all cells became pyknotic by 8 hr, the cords did not shrink for at least 2 days. The cellular kinetics of populations of intact and dead cells, adjacent to and remote from the capillaries of the cords, were examined for up to 60 hr after irradiation and it was found that: (i) before treatment, average LI (adjacent) was 13% and LI (remote) was 2%, (ii) after irradiation, cells expressed pyknosis after passing through the S phase of the cell cycle, so that (iii) at early intervals there was a larger proportional rise in pyknotic cells in the adjacent than the remote zone. However, (iv) at later intervals there was always a higher proportion of dead cells in the remote zone.

Radiotherapy of the rhabdomyosarcoma R1H of the rat: kinetics of cellular inactivation by fractionated irradiation

The kinetics of cellular inactivation by fractionated irradiation in the R1H rhabdomyosarcoma of the rat was studied in the dose range of 1.07 to 12.50 Gy per fraction. Regimens of 1, 3, 5, 7, and 10 fractions per week for several weeks were compared. The number of clonogenic tumor cells per tumor in the course of the different treatment schedules was determined using an in vitro colony assay. The results show that the proliferation of clonogenic tumor cells is decelerated in the course of a fractionated radiotherapy. The deceleration persists for several days after end of treatment, until accelerated repopulation is initiated. The fraction of tumor cells inactivated per week was only dependent on the total dose per week, that is the cellular response was the same whether the weekly dose was applied in 1,3,5,7, or 10 fractions. Thus, the fractionation regimens were considerably more effective than expected from calculations based on single-dose in situ survival curves.

Accelerated fractionation in the radiation treatment of head and neck cancer. A critical comparison of different strategies

There is strong clinical and radiobiological evidence that protraction of overall treatment time has an adverse influence on the radiocurability of certain human tumors. Overall treatment time can be reduced without recourse to large dose fractions by the use of accelerated fractionation, but in patients with head and neck cancer acute mucosal reactions limit the extent to which treatment can be accelerated. Three different prototypical schedules for accelerated fractionation have been devised to avoid exceeding acute mucosal tolerance. Type A consists of an intensive short course in which the overall duration of treatment is markedly decreased with a corresponding substantial reduction of total dose; type B achieves a modest decrease in overall time without reduction of total dose by using a split-course technique; type C also achieves a modest decrease in overall time without reduction of total dose by means of the concomitant boost technique. A hybrid schedule combining features of types B and C allows additional shortening of overall treatment time without reduction of total dose. Available radiobiological and clinical data suggest that schedules of types B or C which do not compromise total dose are generally preferable to those of type A in which there is a trade-off between total dose and overall time. For a given total dose and overall time, a continuous treatment of type C is likely to produce more cell kill than a split-course of type B, although the latter will be better tolerated. Because of the increased acute toxicity associated with all schedules of accelerated fractionation, rational selection of patients for such treatment is important. New techniques to measure the potential doubling time of human tumors in vivo offer this prospect.

Frequencies of independent and simultaneous selection of Chinese hamster cells for methotrexate and doxorubicin (adriamycin) resistance

We have determined the frequency with which Chinese hamster cells become resistant to either methotrexate or doxorubicin (former generic name, adriamycin) alone or to the two drugs simultaneously. We find that the frequency of acquisition of simultaneous resistance is 10-100 times higher than that predicted from the frequency of each resistance selected independently. In approximately 50% of cloned resistant variants, resistance is the result of amplification of the dihydrofolate reductase gene (methotrexate) and/or of the multiple-drug-resistance P-glycoprotein gene (doxorubicin). Prior exposure of cells to hypoxia markedly enhances these resistance frequencies. Our results indicate that the simultaneous emergence of resistance to these two cancer chemotherapeutic agents are not independent events, and we interpret them to constitute two consequences of the same basic process occurring at a high frequency.

Size dependent changes in tumor phosphate metabolism after radiation therapy as detected by 31P NMR spectroscopy

In Vivo 31P NMR spectroscopy was used to study changes in phosphate metabolism that occur after irradiation of the C3H fibrosarcoma, FSaII. Previously, we have shown that small FSaII tumors (less than 250 mm3) have a greater phosphocreatinine/inorganic phosphate (PCr/Pi) ratio and a lower hypoxic cell fraction (HCF) than large FSaII tumors (greater than 250 mm3). Six small tumors (113 +/- 26 mm3) were treated with radiation doses chosen to induce local control in greater than 50% of animals, (70-100 Gy, single fraction). Minimal changes in the tumor 31P NMR spectrum were seen over eight days of monitoring. During this interval, tumor regression began a minimum of 36 hours after radiation. This contrasted with large tumors (650-1000 mm3) wherein a significant increase in the Pcr/Pi ratio was seen 44 hr after irradiation. In tumors of this size range, a tumor growth delay of 4 to 7 days is obtained after a single 70 Gy fraction of radiation. Since small FSaII tumors have a minimal HCF (approximately equal to 4%), radiation induced reoxygenation would not be expected to have a large effect on their average cellular metabolism. Large tumors of this histology have a high HCF (greater than or equal to 40%), and may therefore be expected to have a significant average change in tumor cell metabolism with reoxygenation. The 31P NMR observations of small and large tumors after irradiation are compatible with radiation induced reoxygenation in the larger tumors.

Is misonidazole binding to mouse tissues a measure of cellular pO2?

Misonidazole (MISO), a hypoxic cell radiosensitizer, forms covalently-linked adducts to cellular molecules as a result of bioreductive metabolism, a process which is strongly dependent upon oxygen concentration. MISO binding to liver tissue taken from air-breathing mice was three to five times greater than binding to other normal tissues. The relative binding of [14C]MISO to various mouse tissue cubes in vitro was measured by autoradiography as a function of defined oxygen concentrations, and standard curves (binding rate vs oxygen concentration) were generated. The oxygen concentration for half-maximum binding as well as the maximum and minimum binding rates (grains per 100 micron 2) observed for liver tissue were not significantly different from those measured for brain or heart tissue. These results, along with previously published data on MISO binding to isolated hepatocytes in vitro, suggest that the elevated binding to liver in vivo may result, in part, from the organ existing at a significantly lower pO2 than other normal tissues. They also suggest that this drug adduct procedure could be developed as a sensitive method for the quantitative measurement of tissue pO2 at the cellular level.

Tumour bed effect: hypoxic fraction of tumours growing in preirradiated beds

The reduction in tumour growth rate seen when tumours are implanted into preirradiated sites, the tumour bed effect (TBE), is believed to be due to radiation damage to vascular stroma, leading to defective angiogenesis in the tumour. The present work examined whether or not the functional inadequacy of irradiated stroma was accompanied by an increased hypoxic fraction in tumours growing in irradiated beds. Mouse flank skin was given 0 or 20 Gy X-rays and RIF-1 fibrosarcoma cells were implanted i.d. into the centre of the treatment field one week later. Tumours of 200 mm3 were irradiated under clamped or unclamped conditions and the hypoxic fraction measured from the displacement of the corresponding survival curves, assayed in vitro. Results indicated a small increase in the hypoxic fraction. Averaging values from three independent experiments, the percentage of hypoxic cells increased from 2.5 per cent for cells in tumours growing in unirradiated beds to 4.6 per cent for those from tumours in beds given 20 Gy. Thus an irradiated vascular bed is still to some extent able to maintain the proportion of oxic: hypoxic tumour cells found in tumours growing in unirradiated beds, despite manifest changes in tumour necrosis and growth rate.

Deriving cell survival curves from the overall responses of irradiated tumours: analysis of published data for tumour spheroids

Curves of growth delay (GD) or 'cure' after graded doses of radiation have been analysed for 16 lines of human and animal tumours grown as multicellular spheroids in vitro. Dose-survival curves were derived for those cellular units from which spheroids regrow after unsuccessful irradiation (spheroid-regenerating cellular units, SRU). For 10 sets of data from 6 spheroid lines, the Do's and extrapolation numbers of the SRU derived by GD could be compared with the response of the clonogenic cells of the spheroids. For Do, a good correlation (r = 0.910) was found between the two; this was true also for Do derived from curves of spheroid 'cure' (7 sets of data from 6 spheroid lines) and clonogenic cells (r = 0.986). Using GD, the correlation of extrapolation numbers was less good (r = 0.682), the values for SRU commonly being higher than those for clonogenic cells. This may reflect features of the growth curves of spheroids after the lower range of doses of radiation. For human and animal tumour spheroids of 250 microns or less, derived Do ranged from 0.5 to 2.5 Gy. For spheroids of 350 microns or more, derived Do for animal tumour lines ranged from 3.4 to 4.2 Gy, for human lines from 1.5 to 2.1 Gy.

Effect of temperature on blood flow and hypoxic fraction in a murine fibrosarcoma

The effect of hypo to hyperthermic temperatures on tumor blood flow and hypoxic cell fractions was studied in a murine fibrosarcoma transplanted in the hind leg of anesthetized mice. The blood flow to the tumor was assessed by the determination of the uptake of Thallium-201; the hypoxic cell fraction was estimated from cell survival curves derived from data based on lung colony assay. Over a temperature range of 18 degrees to 46 degrees C, the maximal blood flow occurred at 35 degrees C which was approximately two times greater than that at room temperature (24 degrees C) or at 39 degrees C. The hypoxic cell fraction at 35 degrees C was 11%, and was significantly less than that at 24 degrees C or at 39 degrees C. The hypoxic cell fractions at 24 degrees C and at 39 degrees C were 45% and 32%, respectively. These results suggest that the optimal radiation sensitivity of peripherally located tumors can be obtained by warming the tumors to temperatures where maximal blood flow and minimal hypoxic cell fraction occur.

Chemosensitization by misonidazole in CCNU-treated spheroids and tumours

Misonidazole has been demonstrated to enhance the cytotoxicity of several common antineoplastic drugs in vitro and in vivo, and its mechanism of action as a chemosensitizer, though still unknown, is thought to be dependent upon hypoxia. We have used fluorescence-activated cell sorting to evaluate chemopotentiation by misonidazole as a function of cell position in V79 spheroids and KHT tumours. CCNU toxicity was enhanced in all cell subpopulations of both tumours and spheroids, with greater consistency than might be predicted on the basis of the known variations in oxygen tension. Further, both misonidazole and CCNU as single agents were preferentially toxic in the less well oxygenated regions of each system, arguing that differential toxicity cannot be implicated in the chemopotentiation observed. In fact, increased treatment toxicity did not necessarily lead to increased chemopotentiation, nor was potentiation directly related to the metabolism/binding of the misonidazole. Chemopotentiation in multicell systems thus appears to be a complex, multi-factorial process.

Radiolabeled hypoxic cell sensitizers: tracers for assessment of ischemia

Hypoxic, non-functional, but viable, tissue may exist in heart and brain following an arterial occlusion. Identification of such tissue in vivo is crucial to the development of effective treatment strategies. It has been suggested that certain compounds capable of sensitizing hypoxic tumor cells to killing by x-rays (i.e., misonidazole) might serve as in vivo markers of hypoxic tissue in ischemic myocardium or brain if properly radiolabeled. To this end we have radiolabeled two fluorinated analogs of nitroimidazole based hypoxic cell sensitizers with the 110 minute half-lived positron-emitting fluorine-18. The ability of these tracers to quantitate the presence of hypoxic tissue has been studied in a gerbil stroke model. The in vivo uptake of one of these tracers [F-18]-fluoronormethyoxymisonidazole is dependent on the extent of tissue hypoxia, and thus, appears to have potential as a diagnostic indicator of non-functional but viable tissue when the tracer is used in conjunction with positron emission tomography.

Enhancement of tumor radiation response by the combination of a perfluorochemical emulsion and hyperbaric oxygen

The presence of radioresistant hypoxic cells in tumors is believed to be one of the limiting factors in achieving local tumor control by radiotherapy. Treatment with hyperbaric oxygen during irradiation has been shown to improve the radiation response of many solid tumors in rodents and of some tumors in patients. Intravenous administration of perfluorochemical emulsions combined with oxygen breathing at atmospheric pressure has also been shown to improve the radiation response of several rodent tumors. Theoretical considerations suggest that the combination of a perfluorochemical emulsion and hyperbaric oxygen should be significantly more effective than either agent alone. This hypothesis was tested by examining the radiation response of BA1112 rhabdomyosarcomas growing in WAG/rij-Y rats. Treatment with a perfluorochemical emulsion, Fluosol-DA, plus hyperbaric oxygen (3 Atmospheres O2) significantly increased the radiation response of the malignant cells in these solid tumors. The observed changes in the tumor cell survival curve suggest that the combination of Fluosol-DA and HBO decreases the proportion of severely hypoxic cells in the tumor to less than 1.5% of the original value. The effect of the Fluosol-DA dose and the duration of pretreatment with HBO are described.

Nitroimidazole bioreductive metabolism. Quantitation and characterisation of mouse tissue benznidazole nitroreductases in vivo and in vitro

We have investigated the nitroreduction of the 2-nitroimidazole benznidazole (BENZO) to its corresponding amine by murine normal tissues and tumours. In vivo concentrations of BENZO and its amine metabolite were measured by HPLC 3 hr after BENZO, 2.5 mmoles kg-1 i.p. This gave plasma and tissue BENZO concentrations of 96-160 micrograms ml-1 or g-1. Mouse plasma, KHT and RIF-1 tumour BENZO amine concentrations were very low (0.3-1.4 micrograms g-1); kidney and EMT6 tumours had intermediate levels; and liver contained very high amine levels (approximately 50 micrograms g-1). Three per cent of the BENZO dose was recovered as amine in the 24 hr urine, compared to 5% for the parent compound. Nitroreduction to the amine was demonstrated with liver and tumour preparations under N2 in vitro. The reaction was highly dependent on NADPH, and inhibited extensively in air. With liver microsomes and whole homogenates 2 and 3 moles respectively of BENZO were consumed per mole of amine formed. Inhibitor studies showed that NADPH: cytochrome P-450 (cytochrome c) reductase and cytochrome P-450 were both involved in BENZO reduction, predominantly at early and late reduction steps respectively. Aldehyde oxidase contributed to the cytosolic nitroreduction. Purified buttermilk xanthine oxidase also reduced BENZO to its amine under anaerobic conditions in vitro, but very inefficiently. The apparent Km and Vmax for BENZO amine production by whole liver homogenates were 0.148 mM and 1.45 nmole min-1 mg-1 protein respectively. Tumour homogenates were less active than liver; e.g. Vmax for the KHT tumour was 6-10-fold lower.

Tumor hypoxia: its impact on cancer therapy

The presence of radiation resistant cells in solid human tumors is believed to be a major reason why radiotherapy fails to eradicate some such neoplasms. The presence of unperfused regions containing hypoxic cells may also contribute to resistance to some chemotherapeutic agents. This paper reviews the evidence that radiation resistant hypoxic cells exist in solid tumors, the assumptions and results of the methods used to detect hypoxic cells, and the causes and nature of tumor hypoxia. Evidence that radiation resistant hypoxic cells exist in the vast majority of transplanted rodent tumors and xenografted human tumors is direct and convincing, but problems with the current methodology make quantitative statements about the magnitude of the hypoxic fractions problematic. Evidence that radiation resistant hypoxic cells exist in human tumors is considerably more indirect than the evidence for their existence in transplanted tumors, but it is convincing. However, evidence that hypoxic cells are a significant cause of local failure after optimal clinical radiotherapy or chemotherapy regimens is limited and less definitive. The nature and causes of tumor hypoxia are not definitively known. In particular, it is not certain whether hypoxia is a chronic or a transient state, whether hypoxic cells are proliferating or quiescent, or whether hypoxic cells have the same repair capacity as aerobic cells. A number of new methods for assessing hypoxia are reviewed. While there are still problems with all of the new techniques, some of them have the potential of allowing the assessment of hypoxia in individual human tumors.

Perfluorocarbons as oxygen-transport fluids

1. An overview of the proposed biological applications of perfluorocarbons and their emulsions as oxygen-transport fluids is presented. 2. Aspects of the properties, preparation, composition and physiological assessment of perfluorocarbon emulsions are discussed. 3. The experimental basis for some of the potential therapeutic uses of PFCs in liquid ventilation, treatment of decompression sickness, organ perfusion, oxygenation of ischaemic and malignant tissues, and as contrast media for NMR imaging is described. 4. The extent to which emulsified perfluorocarbons may have value as substitutes for red blood cells is discussed in detail. Data from both animal and human studies with such emulsions is reviewed. Brief consideration is also given to the possible use of native and modified haemoglobin in blood replacement together with recent work on the preparation of so-called "synthetic red cells".

Effect of Fluosol-DA/O2 on the antitumor activity and pulmonary toxicity of bleomycin

The effect of an oxygen-carrying perfluorochemical emulsion on bleomycin antitumor activity and pulmonary toxicity was examined. Fluosol-DA (0.3 ml/mouse, i.v.), combined with bleomycin (10 or 15 mg/kg, i.p.) and a 2 h exposure to 95% oxygen (BFO) increased by five- to six-fold the tumor growth delay of FSaIIC fibrosarcoma compared to bleomycin alone (B). Only a slight increase in tumor growth delay was noted with the incomplete combinations of bleomycin and O2 (BO) and bleomycin and Fluosol-DA (BF). When bleomycin (10 mg/kg) was co-administered with 0.3 ml Fluosol-DA and 95% O2, cell survival was reduced ten-fold compared to that seen with bleomycin alone. In contrast, the surviving fraction of cells obtained from FSaIIC tumors treated in vivo indicated that 0.3 ml Fluosol-DA per mouse or a 2 h exposure to 95% O2 did not markedly alter the effects of bleomycin alone. The pulmonary effects of the BFO combination were assessed during the course of the therapy by bronchoalveolar lavage (BAL) analysis and pulmonary hydroxyproline (OH-Pro) content. Mice treated with this combination had a 20-fold increase in total numbers of cells obtained in the BAL compared to control animals. An increased cellularity in the lungs was also seen morphologically. The composition of the cells in the lavage fluid was altered after BFO but not BO treatment and reflected a neutrophilic influx. Furthermore, total protein recovered in the BAL fluid was increased 5-fold in the BFO treatment group compared to that in the control mice. Pulmonary OH-Pro, an index of collagen and fibrosis, was unaffected acutely after three treatments of either BFO or BO compared to control mice. Thus, Fluosol-DA and O2 can enhance the antitumor activity of bleomycin. The increased pulmonary cellularity suggests that this combination may also have adverse effects on lung tissue.

Reoxygenation of the RIF-1 tumor after fractionated radiotherapy

The regrowth delay assay was used to assess hypoxic fraction in the RIF-1 tumor. Results approximating those of earlier paired survival curve data were obtained for previously untreated tumors and tumors treated with a single dose of 15 Gy. Further studies showed that after 5 daily fractions of 5 Gy, the hypoxic fraction returned to approximate pretreatment values within 24 hr.

A novel technique for measuring human tissue pO2 at the cellular level

Some electron-affinic drugs, developed as hypoxic cell radiosensitizers, become selectively bound to the molecules of hypoxic cells by metabolism. This technique has been used to identify zones of chronically hypoxic cells in multicellular spheroids and animal tumours. Tritiated-misonidazole was administered to a patient with advanced melanoma 22 h prior to the surgical resection of a large metastatic s.c. lesion growing on the face. Autoradiographic analysis of histological sections revealed zones of intense labelling by the radioactive drug, indicative of tumour cells which were chronically hypoxic. This technique appears to provide an indirect measurement of tissue pO2 at the cellular level from which estimates of the tumour hypoxic fraction can be made. These data are encouraging as regards the development of 'sensitizer-adduct' procedures for the invasive and non-invasive measurement of hypoxia in both tumours and normal tissues.