Improvement in RBC flux, acidosis and oxygenation in tumour microregions by Fluosol-DA 20%

Mini-review: Perfluorocarbons, Oxygen Transport, and Microcirculation in Low Flow States: in Vivo and in Vitro Studies

The in vivo study of microvascular oxygen transport requires accurate and challenging measurements of several mass transfer parameters. Although recommended, blood flow and oxygenation are typically not measured in many studies where treatments for ischemia are tested. Therefore, the aim of this communication is to briefly review cardinal aspects of oxygen transport, and the effects of perfluorocarbon (PFC) treatment on blood flow and oxygenation based mostly on studies performed in our laboratory. As physiologically relevant events in oxygen transport take place at the microvascular level, we implemented the phosphorescence quenching technique coupled with noninvasive intravital videomicroscopy for quantitative evaluation of these events in vivo. Rodent experimental models and various approaches have been used to induce ischemia, including hemorrhage, micro- and macroembolism, and microvessel occlusion. Measurements show decrease in microvascular blood flow as well as intravascular and tissue oxygen partial pressure (PO2) after these procedures. To minimize or reverse the effects of ischemia and hypoxia, artificial oxygen carriers such as different PFCs were tested. Well-defined endpoints such as blood flow and tissue PO2 were measured because they have significant effect on tissue survival and outcome. In several cases, enhancement of flow and oxygenation could be demonstrated. Similar results were found in vitro: PFC emulsion mixed with blood (from healthy donors and sickle cell disease patients) enhanced oxygen transport. In summary, PFCs may provide beneficial effects in these models by mechanisms at the microvascular level including facilitated diffusion and bubble reabsorption leading to improved blood flow and oxygenation.

Effects of the Perfluorochemical Emulsion FMIQ on the Radiation Response of EMT6 Tumours

The effects of FMIQ, a perfluorochemical emulsion based on perfluoro-N-methyldecahydroisoquinoline, were examined using BALB/c mice and EMT6 mammary carcinomas. The radiobiological effects of FMIQ were similar to those found previously for Fluosol in the same tumour/host system. Although the perfluorochemical content (20% w/v) and oxygen-carrying capacity of FMIQ are similar to those of Fluosol, the formulation of FMIQ offers some advantages over that of Fluosol. For example, FMIQ has greater stability during storage. FMIQ also is formulated without pluronic F-68 and is based on a perfluorochemical (FMIQ) having a shorter tissue dwell time than the perfluorotripropylamine in Fluosol; it therefore may produce fewer side-effects than Fluosol. The lifetime of the circulating perfluorochemical droplets in BALB/c mice was longer than FMIQ than for Fluosol; this could offer an advantage in fractionated radiotherapy. These findings give reason to expect that FMIQ may prove to be a better emulsion than Fluosol for clinical use as an adjunct to cancer therapy.

Oxygenation in tumors by modified hemoglobins

The effect of systemic injection of modified hemoglobin (Hb) prepared from bovine, human, or mouse Hb on tumor oxygenation was investigated. Hb was modified by (1) diisothiocyanatobenzenesulfonate (DIBS) to yield cross-linking within a tetramer; (2) glycolaldehyde (Glyal) to yield cross-linking between and within tetramers; (3) carboxymethylation (Cm) to change oxygen affinity; or (4) poly(ethylene glycol) (PEG) to yield attachment between tetramers. HGL9 (human glioma) in nude mice and FSaII (mice fibrosarcoma) in C3H mice were used as tumor models. Dose and time dependency were detected in the oxygenation effect by bovine-PEG-Hb. Internal cross-linkage prolonged the half-life in the circulation, and thus showed a significant effect. Compared to bovine-CmHb, bovine-DIBS-Hb and bovine-DIBS-CmHb were more effective. Decreasing the oxygen affinity by Cm significantly enhanced tumor oxygenation. Human-DIBS-CmHb was more effective than human-DIBS-Hb. These effects were caused by oxygen carrying capacity of modified Hbs as well as hemodynamic factors, and the injection seemed to reduce both perfusion-limited (acute) and diffusion-limited (chronic) hypoxia.

Tumour oxygenation, radiosensitivity, and necrosis before and/or after nicotinamide, carbogen and perflubron emulsion administration

Hypoxia is one of the factors involved in tumour resistance to radiotherapy. One way to improve tumour oxygenation is to use oxygen carriers such as perflubron emulsion plus carbogen or vasoactive drugs such as nicotinamide. The perflubron emulsion and carbogen act mainly on hypoxia caused by limited diffusion of oxygen; nicotinamide acts mainly on acute hypoxia. The aim was to correlate radiosensitivity and pO2 measurements (computerized pO2 histograph) after nicotinamide, perflubron emulsion and carbogen administration, and to determine the role of necrosis in this correlation. Two human tumour xenografts (HRT18, Na11 +) and one rodent tumour (EMT6) were used. Clonogenic assays and pO2 measurements were performed under similar conditions. The radiosensitization and oxygenation levels increased with all treatments. The maximal effects were found with the combination of nicotinamide (1 g/kg), perflubron emulsion and carbogen. A correlation between the radiosensitization and the pO2 measurements was found for the three cell lines with a cut-off point of 10 mmHg. The presence of necrosis could explain the low pO2 (< 2 mmHg) found even when complete radiosensitization was observed.

Fluorocarbon emulsions

Perfluorocarbon emulsions have been the topic of intense investigation for many years and presently there are still no absolute indications for their use in clinical practice. The relatively disappointing results of the early clinical studies, as a consequence of using low concentrations of a relatively underdeveloped emulsion, have been responsible for a largely negative impression and it is now essential that the newer second generation emulsions should be judged individually with regard to their efficacy and toxicity under different circumstances. Technological advancement in the fields of chemistry and detergent/emulsifier research will continue and new formulations are being developed which which will require to be tested in models in the laboratory. In the future, this class of drugs will continue to be the topic of intense investigation and their mechanisms of action, which are undoubtedly more complex than the simple carriage of dissolved gases in solution, will be clarified. However, whether fluorocarbon emulsions will ever be used as a 'blood substitute' as was originally anticipated is doubtful.

Changes in tumour blood flow, oxygenation and interstitial fluid pressure induced by pentoxifylline

Pentoxifylline (PTX) has been shown to increase radiation damage to tumours and to decrease late radiation-induced injury to normal tissues. This tumour radiation sensitisation results from increased oxygen supply via improved tumour perfusion. We propose that the improved perfusion results from decreased viscous resistance and/or geometric resistance. The decreased flow resistance may be accompanied by a reduction in microvascular pressure (MVP). Since MVP is approximately equal to the interstitial fluid pressure (IFP), PTX should lead to a decrease in IFP. To test this hypothesis, we measured PO2, laser Doppler flow (RBC flux) and IFP in FSaII murine tumours at two doses (PTX at 25 and 100 mg per kg body weight) which sensitise this tumour to X-irradiation. We found that 25 mg kg-1 PTX was ineffective, but 100 mg kg-1 PTX was effective in increasing the PO2 of this tumour. PTX at 100 mg kg-1 (i.p.) increased median PO2 from 5 to 7 mmHg (P < 0.05) within 2 h, and decreased the fraction of PO2 values < 5 mmHg from 65% to 45% (P < 0.05). In support of our hypothesis, we found that with this dose of PTX, RBC flux in the tumour centre increased significantly (n = 6, P < 0.05) prior to an approximately 40% decrease (n = 13, P < 0.05) in tumour interstitial fluid pressure (TIFP), without changes in mean arterial blood pressure (MABP). In conclusion, a single i.p. administration of PTX at 100 mg kg-1 can increase oxygen availability in the tumour due to ameliorate hypoxia in tumour microregions. Second, PTX can lower the elevated TIFP without lowering the MABP.

Nicotinamide and carbogen: relationship between pO2 and radiosensitivity in three tumour lines

The effects of carbogen breathing, nicotinamide injection and their combination on tumour radiosensitivity were correlated with changes in tumour O2 tension to determine the relationship between radiosensitivity and measured pO2. The radiosensitivity (in vivo-in vitro colony assay) and O2 tension (computerized pO2 histograph KIMOC 6650) of two human xenografted tumours (HRT18 and Na11+) and one murine tumour (EMT6) were measured under similar experimental conditions. A single dose of radiation was delivered (8 Gy for HRT18 and Na11+, 12 Gy for EMT6). Carbogen breathing, nicotinamide injection, and their combination all significantly increased radiosensitivity in the three cell lines (p < 0.05); the most efficient treatment was carbogen plus nicotinamide. The radiosensitization was optimum for EMT6 and Na11+. Mean and median pO2 increased with all three treatments, except for carbogen breathing in EMT6. Carbogen breathing had little effect on the proportion of low pO2 values, but induced pO2 values > 30 mmHg in all three tumour lines. Nicotinamide decreased the proportion of low pO2 values. This effect is larger with the combination carbogen plus nicotinamide. Almost all pO2 values < 2.5 mmHg were eliminated for HRT18 and EMT6. The relationship between radiosensitization and pO2 was significant when pO2 was expressed as the percentage of values below 7, 8, 9 and 10 mmHg for the three lines.

Radiosensitivity, blood perfusion and tumour oxygenation after perflubron emulsion injection

The effect of 90% and/or 100% w/v perflubron (perfluorooctyl bromide (PFOB); Alliance Pharmaceutical Corp.) emulsions on radiosensitivity, tumour relative perfusion and oxygenation was studied using EMT6 tumours in nude mice. Perflubron (2-15 ml/kg) emulsion was injected. The mice inhaled carbogen for 30 min and 60 min prior to irradiation. The radiosensitizing effect of the 90% w/v emulsion was maximal at 4 ml/kg. The tumour relative perfusion diminished after injection of both 100% and 90% w/v emulsions in carbogen-breathing mice at a dose of 15 ml/kg. This drop could explain the lack of efficiency of these treatments at this high concentration. Lastly, tumour oxygenation was increased after administration of perflubron emulsion plus carbogen.

Modulation of tumor oxygenation and radiosensitivity by a perfluorooctylbromide emulsion

The effect of a concentrated perfluorooctylbromide emulsion (Oxygent) on the radiosensitivity and oxygenation of solid tumors was examined using EMT6 mammary tumors in BALB/c mice and BA1112 rhabdomyosarcomas in WAG/rij rats. Treatment with Oxygent plus carbogen or oxygen breathing increased the radiosensitivity of both tumors. Analysis of tumor cell survival data and polarographic measurements of intratumoral pO2 indicated that this potentiation reflected an increase in the proportion of well-oxygenated tumor cells. Treatments with carbogen breathing alone, with Oxygent plus air-breathing, or with a vehicle emulsion containing all the components except the perfluorocarbon did not produce comparable improvements in tumor radiosensitivity. Concentrated perfluorooctylbromide emulsions appear to warrant further development and preclinical testing as adjuncts to cancer therapy.