Influence of the 100% w/v perfluorooctyl bromide (PFOB) emulsion dose on tumour radiosensitivity

Perfluorocarbon nanodroplets can reoxygenate hypoxic tumors in vivo without carbogen breathing

Nanoscale perfluorocarbon (PFC) droplets have enormous potential as clinical theranostic agents. They are biocompatible and are currently used in vivo as contrast agents for a variety of medical imaging modalities, including ultrasound, computed tomography, photoacoustic and 19F-magnetic resonance imaging. PFC nanodroplets can also carry molecular and nanoparticulate drugs and be activated in situ by ultrasound or light for targeted therapy. Recently, there has been renewed interest in using PFC nanodroplets for hypoxic tumor reoxygenation towards radiosensitization based on the high oxygen solubility of PFCs. Previous studies showed that tumor oxygenation using PFC agents only occurs in combination with enhanced oxygen breathing. However, recent studies suggest that PFC agents that accumulate in solid tumors can contribute to radiosensitization, presumably due to tumor reoxygenation without enhanced oxygen breathing. In this study, we quantify the impact of oxygenation due to PFC nanodroplet accumulation in tumors alone in comparison with other reoxygenation methodologies, in particular, carbogen breathing. Methods: Lipid-stabilized, PFC (i.e., perfluorooctyl bromide, CF3(CF2)7Br, PFOB) nanoscale droplets were synthesized and evaluated in xenograft prostate (DU145) tumors in male mice. Biodistribution assessment of the nanodroplets was achieved using a fluorescent lipophilic indocarbocyanine dye label (i.e., DiI dye) on the lipid shell in combination with fluorescence imaging in mice (n≥3 per group). Hypoxia reduction in tumors was measured using PET imaging and a known hypoxia radiotracer, [18F]FAZA (n≥ 3 per group). Results: Lipid-stabilized nanoscale PFOB emulsions (mean diameter of ~250 nm), accumulated in the xenograft prostate tumors in mice 24 hours post-injection. In vivo PET imaging with [18F]FAZA showed that the accumulation of the PFOB nanodroplets in the tumor tissues alone significantly reduced tumor hypoxia, without enhanced oxygen (i.e., carbogen) breathing. This reoxygenation effect was found to be comparable with carbogen breathing alone. Conclusion: Accumulation of nanoscale PFOB agents in solid tumors alone successfully reoxygenated hypoxic tumors to levels comparable with carbogen breathing alone, an established tumor oxygenation method. This study confirms that PFC agents can be used to reoxygenate hypoxic tumors in addition to their current applications as multifunctional theranostic agents.

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.

Fluorocarbon nanoparticles as multifunctional drug delivery vehicles

The history and current status of fluorocarbon nanoparticles in biomedicine is briefly reviewed. The deficiencies of current fluorocarbon nanoparticle formulations are highlighted. Strategies to remedy such deficiencies and to functionalize fluorocarbon nanoparticles are presented. Potential applications of fluorocarbon nanoparticles as multifunctional drug delivery vehicles are discussed. The strength of fluorocarbon nanoparticles as drug delivery vehicles is that they integrate drug delivery with non-invasive MR imaging so that the biodistribution of the pharmaceutical entity (drug + delivery vehicle) can be monitored in real time. This, in turn, permits the physician to adjust treatment plan for each patient based on his/her actual response to the ongoing treatment.

Accuracy of fluorocrit in determination of blood perflubron concentration

Previous studies examining the radiosensitizing effects of perfluorochemical emulsions have based dose recommendations on a measurement known as fluorocrit. The fluorocrit is the proportion of blood volume occupied by perfluorochemicals and is measured using standard hematocrit procedures. This measurement is inherently crude and subject to error and variability between different individuals measuring the same sample. Furthermore, the fluorocrit method has not been compared to other quantitative methods to determine its reliability. The purpose of this study was to compare fluorocrit measurements to those obtained by gas chromatographic analysis. A 90% w/v perflubron emulsion was administered to six normal dogs once weekly for four weeks and peripheral blood samples were obtained at specified time points for analysis. A total of 123 blood samples were analyzed by both methods. The relationship between blood fluorocrit and plasma perflubron concentration measured by gas chromatography was examined using regression models. Based on the modest predictive value (r2 = 0.3683) of the derived statistical model, we conclude that fluorocrit measurement is an inaccurate method of estimation of blood perflubron concentration. Caution must, therefore, be exercised when extrapolating data and dose recommendations from reports of studies using flurocrit as the only estimate of blood perflubron concentration.

Efficacy of agents counteracting hypoxia in fractionated radiation regimes

BACKGROUND AND PURPOSE

Solid tumours contain hypoxic cells which are resistant to radiotherapy. This study compares the efficacy of several strategies to counteract diffusion-limited hypoxia, or intermittent hypoxia in a fractionated regimen of 1 to 6 x 2 Gy.

MATERIALS AND METHODS

Nicotinamide (250 mg/kg), perflubron emulsion (Oxygent) (4 ml/kg), tirapazamine (SR4233) (0.10 mmol/kg) and carbogen breathing, administered alone or in combination, were investigated on two tumour cell lines: EMT6 (a rodent mammary carcinoma) and HRT18 (a human rectal adenocarcinoma) using a clonogenic assay. The radiosensitizing effect of the agents was assessed after 1 and 6 x 2 Gy for drugs used alone, and 1, 2, 4, 6 x 2 Gy for drugs used in combination.

RESULTS

At the end of the fractionated radiation regimen, the combination of nicotinamide + carbogen induced the greatest radiosensitization for EMT6 tumours, while greatest radiosensitization of HRT18 was obtained with nicotinamide + carbogen + tirapazamine.

CONCLUSION

The efficacy of the strategies for overcoming hypoxia using a fractionated regimen depends on the tumour cell line. These differences could be linked to differences in the initial percentages of acute and chronic hypoxic cells, and to changes in the two types of hypoxia during treatment.

Interstitial fluid, plasma protein, colloid, and leukocyte uptake into initial lymphatics

Lymphatics serve to remove from the interstitium a range of materials, including plasma proteins, colloid materials, and cells. Lymph flow rates can be enhanced by periodic tissue compression or venous pressure elevation, but little is known to what degree enhancement of lymph flow affects material transport. The objective was to examine the uptake of plasma proteins, a colloidal perflubron emulsion (LA-11063, mean particle diameter = 0.34 micron), and leukocytes into lymphatics. Prenodal collecting lymphatics in the lower hindlimb of rabbits were cannulated with and without foot massage and after elevation of venous pressure (40 mmHg). The average lymph flow rates were elevated approximately 22-fold by the skin massage but only about threefold by venous pressure elevation. Lymph-to-plasma protein concentration ratio remained unchanged by the massage but decreased significantly after venous pressure elevation. Lymph colloid concentration and leukocyte counts were elevated on average 47 and 8.5 times, respectively, by foot massage, but both decreased after venous pressure elevation. These results suggest that skin movement by massage and elevation of the venous pressure lead to opposite lymph transport kinetics of protein, colloids, and cells. Massage is more effective to enhance material transport out of the interstitium into the initial lymphatics.

Transient perfusion and radiosensitizing effect after nicotinamide, carbogen, and perflubron emulsion administration

In order to improve the effect of radiation on tumour response, nicotinamide, perflubron emulsion and carbogen were administered which act on both diffusion limited hypoxia and intermittent perfusion limited hypoxia. These treatments were used in different combinations. The maximal radiosensitizing effect was found with the combination of the three treatments. The aim of this study was to use a double staining method (Hoechst 33342 and DiOC7(3) to evaluate the influence of nicotinamide, perflubron emulsion and carbogen on transient perfusion in three tumour cell lines transplanted onto nude mice: one rodent (EMT6), two human (HRT18, a rectal adenocarcinoma; and Na11+, a melanoma). For untreated groups, the percentage of closed and mismatched vessels depended on the tumour cell line. Carbogen alone or carbogen plus perflubron emulsion decreased the number of mismatched and closed vessels only for the two human cell lines. Nicotinamide was effective in decreasing the percentage of mismatched and closed vessels only for the melanoma cell line. The combination of nicotinamide, carbogen and perflubron emulsion was the most effective at decreasing both percentage of mismatched and closed vessels in all three tumours studies. This combination was also the most effective at enhancing the radiation response in all three tumours.

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.

Intracellular and extracellular transport of perfluoro carbon emulsion from subcutaneous tissue to regional lymphatics

The aim of this study was to examine the transport mechanism of colloidal particles from subcutaneous tissue to lymphatics. The mechanism of lymph transport was studied in leg prenodal lymphatics of anesthetized rabbits using Imagent LN (60% W/V perfluorooctyl bromide [PFOB] emulsion). Extracellular (dispersed particles) and intracellular (phagocytosed particles by macrophages) PFOB transport was measured in lymph fluid after 0.1ml injection of fluorescently labeled PFOB emulsion into the dorsal skin of the rabbits' foot. Samples were collected from cannulated lower leg prenodal lymphatics. Particles of PFOB emulsion were observed by using a fluorescent technique. The foot/leg were moved passively in a rotary direction at 0.3Hz. Extracellular and intracellular PFOB could be determined in the lymph samples. These findings indicate that both intracellular and extracellular transport mechanisms play a role in the uptake of colloidal particles from interstitial tissue to lymphatics.

Pharmacokinetics and tolerance of weekly OXYGENT CA infusions in the dog

This study determined the OXYGENT CA (90% w/v perflubron emulsion, Alliance Pharmaceutical Corporation) dose necessary to achieve a 3-4% fluorocrit, and the tolerance of this dose administered once per week for four weeks to dogs. This study simulated OXYGENT CA use as a radiosensitizing agent. Six adult dogs were administered 6 ml/kg OXYGENT CA once per week for 4 weeks. Blood samples were collected following infusion, until fluorocrits were < or = 0.5%. One week after the fourth infusion, three dogs were necropsied. Liver biopsies were obtained from the remaining three dogs which were monitored 12 additional weeks. All dogs achieved fluorocrits > 3.0% (3.5-5.1%) with the 6 ml/kg dose. A 3 ml/kg dose did not provide a fluorocrit > 3.0%. Serum bilirubin concentrations were elevated at 24-hour sampling times and declined within 72 hours. Elevations in ALT, SAP, and bile acids were noted. Splenic and hepatic microvasculature fibrosis occurred in the long-term study dogs. Thrombocytopenia occurred in 5/6 dogs, necessitating exclusions of one dog from 2 infusions. However, 3/5 thrombocytopenic dogs had titers for Ehrlichia sp., which elicits thrombocytopenia. Therefore, we cannot conclude the effect of OXYGENT CA on platelets.

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.

[Fluorocarbon emulsions as injectable oxygen carriers. Recent progress and perspectives]

Fluorocarbon emulsions are presently being developed to serve as injectable oxygen carriers (so-called "blood substitutes"). In this approach oxygen is simply dissolved in the liquid carrier and the amount of gas dissolved is proportional to its partial pressure. Increasing the O2-delivering capacity is therefore achieved more easily by increasing the oxygen content of the air breathed by the patient than by increasing the dose of fluorocarbon administered. The absence of a chemical bond between the gas and its carriers allows over 90% of the transported oxygen to be delivered. The fluorocarbon droplets act as oxygen carriers , and also appear to facilitate its diffusion. Chemically and biologically highly inert, fluorocarbons are excreted by exhalation without being metabolized. The first generation of emulsions, exemplified by Fluosol has only limited efficacy due to its low fluorocarbon content, low intravascular persistence and insufficient stability. It has to be stored and distributed frozen, then reconstituted prior to use. Fluosol has nevertheless been licensed by the Food and Drug Administration for use during high risk PTCA. The second generation of injectable fluorocarbon emulsions, exemplified by Oxygent is 4-5 times more concentrated and consequently more efficacious than Fluosol. Considerably more stable, this emulsion can be stored for over one year at 5-8 degrees C and is ready for use. The fluorocarbon used has a significantly shorter organ-retention time. The applications of the present emulsions are still limited by their short intravascular persistence, and are those for which prolonged efficacy is not required, which includes perioperative hemodilution, ischemia, cardioplegia, reperfusion, sensitization of tumors to radio- and chemotherapy, organ preservation and diagnosis. The efficacy of the new emulsions has been established in various animal models. The mild side-effects observed in Phase I clinical trials appear to result from a transient activation of the monocyte/macrophage system and to be suppressed prophylactically by cyclooxygenase inhibitors or corticosteroids. Research is presently oriented towards controlling intravascular persistence better, increasing emulsion stability further, minimizing side-effects and optimizing emulsion characteristics for specific indications.

Incorporation of new amphiphilic perfluoroalkylated bipyridine platinum and palladium complexes into liposomes: stability and structure-incorporation relationships

New perfluoroalkylated side-chain (formed by a perfluoroalkyl tail grafted onto a hydrocarbon spacer) bipyridine complexes of platinum, palladium and their hydrocarbon analogs, when dispersed in aqueous solutions in the presence of egg yolk phospholipids (EYP), are incorporated into EYP vesicles. Most complexes retain their chemical and structural integrity when entrapped into liposomes, thermally sterilized under the FDA norms and stored for 3 months at 25 degrees C. All complex/liposome preparations consist mainly of small unilamellar vesicles (size less than 100 nm) together with a population of larger uni- or multilamellar vesicles (100 to 230 nm). These preparations are remarkably stable with respect to particle size and size distribution evolution and complex leakage: no precipitate of drug was detected even after 7 months of storage at 25 degrees C. The impact of the perfluoroalkyl tail and of the other structural features of the complexes on their incorporation efficiency into EYP liposomes is assessed. A high fluorophilic character (long perfluoroalkyl tail), when tempered by an equally lipophilic one (long hydrocarbon spacer) is not detrimental to the incorporation efficiency of a perfluoroalkylated drug into hydrocarbon vesicles. This incorporation efficiency is considerably improved by the introduction of a double bond between the perfluoroalkyl tail and the hydrocarbon spacer, forming the bipyridine side-chains.

Magnetic resonance spectroscopy studies on experimental murine and human tumors: comparison of changes in phosphorus metabolism with induced changes in vascular volume

The responses of two experimental murine tumors and two human tumor xenografts to the vasodilator hydralazine were compared using two magnetic resonance spectroscopy endpoints. Changes in tumor metabolism were determined using 31P MRS where inorganic phosphate levels relative to total phosphate (Pi/total) were measured, and alteration in tumor blood volume was examined using 19F MRS with perfluorooctylbromide (PFOB) as tracer. The integrated 19F signal from PFOB is dose dependent and stable for at least 2 hr after injection. The murine tumors SCCVII/Ha and KHT both showed changes in tumor metabolism after hydralazine, as an increase in Pi/total. However, hydralazine reduced vascular volume in the KHT tumor, demonstrated by reduced 19F signal from PFOB, but no such reduction was seen in the SCCVII/Ha tumor. In contrast, hydralazine had no effect on phosphorus metabolism in the HT29 and HX118 human tumor xenografts, but reduced vascular volume in both tumors. These results demonstrate that the effects of vasoactive agents such as hydralazine on tumor phosphorus metabolism are only partially consistent with changes in vascular volume, measured by the 19F MRS technique.

Effects of hyperbaric oxygen and a perfluorooctylbromide emulsion on the radiation responses of tumors and normal tissues in rodents

Perfluorochemical emulsions are being examined in many laboratory and clinical studies as possible adjuncts to radiotherapy and chemotherapy. The studies reported here examine the clinical potential of hyperbaric oxygen (HBO) in combination with a highly concentrated perfluorochemical emulsion (Oxygent) containing 100% w/v perfluorooctylbromide (PFOB). HBO alone produced only a small improvement in the radiation response of BA1112 tumors in WAG/rij rats, while regimens combining HBO with Oxygent produced much greater radiation sensitization. A sham emulsion, formulated without the O2-carrying PFOB, did not alter the radiation response of the tumors in comparison with that seen with HBO alone. Neither HBO nor Oxygent plus HBO altered the radiosensitivity of bone marrow progenitor cells in BALB/c mice. HBO alone augmented skin reactions in BALB/c mice, but addition of Oxygent did not alter the skin reactions in comparison to those seen with HBO alone. Regimens combining Oxygent with HBO selectively increased the radiation sensitivity of tumors relative to normal tissues, thereby enhancing the therapeutic ratio. These results support the potential usefulness of perfluorochemical emulsions and HBO in clinical radiation therapy.

Overview of progress in the fluorocarbon approach to in vivo oxygen delivery

The development of fluorocarbon-based oxygen carriers has experienced rapid progress over the past few years. Fluosol has been approved for use during percutaneous transluminal coronary angioplasty (PTCA) for high-risk patients. Its clinical evaluation is being pursued as an adjunct to cancer therapy and for treatment of myocardial infarction in conjunction with thrombolytic therapy. O2-delivery efficacy has been achieved with the development of the new highly concentrated (4 to 5 times more concentrated than Fluosol), fluid, emulsions of perfluorooctyl bromide (perflubron), trade-named Oxygen. The stability of fluorocarbon emulsions has also improved considerably and the new emulsions can be stored unfrozen and are ready for use. The side-effect profile of these emulsions has been characterized as being the normal response of the body's phagocytes to the injection of particles, a response that is considered physiological rather than pathological in nature; it involves some products of arachidonic acid metabolism and can be controlled pharmacologically. Means of further stabilizing fluorocarbon emulsions, involving molecular-diffusion-controlling additives or fluorinated surfactants, including mixed fluorocarbon-hydrocarbon compounds, have been devised. Increased control over in vivo particle recognition, intravascular persistence and side effects, and at adapting emulsion characteristics to specific applications, is being investigated. The range of therapeutic applications is expanding. The concentrated emulsions will be able to serve as a temporary red blood cell substitute in many situations. Acute normovolemic hemodilution with fluorocarbon emulsions, used in conjunction with homologous predonation and other blood-sparing techniques, should afford greater flexibility, increase the margin of safety, and reduce or alleviate the need for autologous blood transfusion during surgical procedures. Fluorocarbon applications in the cardiovascular field include use during PTCA, for cardioplegia and reperfusion, and the treatment of myocardial infarction. Significant tumor growth delay has been achieved when concentrated emulsions are used in conjunction with cancer radio- or chemotherapy. Liquid ventilation has potential as a unique treatment for the adult and infant respiratory distress syndromes and for drug delivery. The radiopaque and versatile perflubron can also be used in contrast agents for diagnosis with computed X-ray tomography, magnetic resonance imaging and ultrasound, allowing the early detection and staging of cancer. Other potential applications investigated include the treatment of cerebral ischemia, organ and limb preservation, use as a tamponade during retinal repair, etc.

Preclinical evaluation of Oxygent as an adjunct to radiotherapy

These studies examine the potential value of a concentrated emulsion of perfluorooctylbromide (perflubron; Oxygent, Alliance Pharmaceutical Corp.) as an adjunct to radiotherapy. The effects of Oxygent on solid tumors were examined using EMT6 mammary tumors in BALB/c mice and BA1112 rhabdomyosarcomas in WAG/rij rats. Treatment with Oxygent plus O2, carbogen (95% O2/5% CO2), or hyperbaric oxygen (HBO) increased the effects of radiation on the tumors. Analyses of tumor cell survival curves and measurements of intratumor pO2 showed that this potentiation reflected an increase in the proportion of well-oxygenated tumor cells. Neither treatment of the animals with carbogen, O2, or HBO alone nor treatment of air-breathing rodents with Oxygent produced changes of similar magnitude. Treatment with a vehicle emulsion containing all the components of Oxygent except the perflubron did not alter tumor radiosensitivity, showing that tumor radiosensitization required the oxygen-transporting perfluorocarbon, and did not result from any biologic or physiologic effects of other components of the emulsion. These studies also examined the effects of Oxygent on the radiation responses of mouse skin and bone marrow. Oxygent selectively increased the radiation sensitivity of tumors relative to these normal tissues, thereby increasing the therapeutic ratio and producing therapeutic gain. Oxygent appears to warrant further testing as an adjunct to cancer therapy.

Fluorocarbon-based in vivo oxygen transport and delivery systems

The approval of Fluosol, a fluorocarbon emulsion for oxygenating the myocardium during the transluminal coronary angioplasty procedure, is a landmark in the field of injectable oxygen carriers, the so-called blood substitutes. This review discusses the advances made since this first emulsion was initially developed about 12 years ago. Attention is focused on the progress achieved in the preparation and selection of new, better-defined and faster-excreted fluorocarbons, and better surfactants, improved emulsions, knowledge of structure/property relationships along with an improved understanding of the physiologic response to their administration. These advances have led to the development of a second generation of highly concentrated, fluid and stable injectable oxygen carriers suitable for a broad range of clinical applications. Prospects for further progress and future generations of emulsions are also outlined.

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.

Tumor hypoxia, reoxygenation and oxygenation strategies: possible role in photodynamic therapy

The concept of hypoxia and its role in tumor therapy are currently under re-evaluation. Poor oxygenation is no longer visualized as an independent feature promoting necrosis and resistance to treatments, but rather as one of the several interdependent microenvironmental parameters associated with impaired blood perfusion. Tumor cells display several survival strategies and remain clonogenic for long periods in nutrient-deprived situations. Reoxygenation may cause lethal damage, improve the response to therapy, or else allow the cell variants adapted to hypoxia to resume proliferation with enhanced aggressiveness and resistance to treatment. The blood supply parameters, oxygenation status and metabolism of malignant cells are discussed here from the standpoint of tumor photodynamic therapy. The role of the tumor interstitial fluid as oxygen- and sensitizer-carrier is discussed. Techniques for assessing tumor oxygenation and for mapping hypoxic territories are described. Strategies for locally improving the oxygenation levels or for selectively destroying the hypoxic populations are outlined.