Synthetic oxygen transport fluids based on perfluorochemicals: applications in medicine and biology

Perfluorocarbon-based oxygen carriers: from physics to physiology

Developing biocompatible, synthetic oxygen carriers is a consistently challenging task that researchers have been pursuing for decades. Perfluorocarbons (PFC) are fascinating compounds with a huge capacity to dissolve gases, where the respiratory gases are of special interest for current investigations. Although largely chemically and biologically inert, pure PFCs are not suitable for injection into the vascular system. Extensive research created stable PFC nano-emulsions that avoid (i) fast clearance from the blood and (ii) long organ retention time, which leads to undesired transient side effects. PFC-based oxygen carriers (PFOCs) show a variety of application fields, which are worthwhile to investigate. To understand the difficulties that challenge researchers in creating formulations for clinical applications, this review provides the physical background of PFCs’ properties and then illuminates the reasons for instabilities of PFC emulsions. By linking the unique properties of PFCs and PFOCs to physiology, it elaborates on the response, processing and dysregulation, which the body experiences through intravascular PFOCs. Thereby the reader will receive a scientific and easily comprehensible overview why PFOCs are precious tools for so many diverse application areas from cancer therapeutics to blood substitutes up to organ preservation and diving disease.

Enhanced antitumor efficacy in colon cancer using EGF functionalized PLGA nanoparticles loaded with 5-Fluorouracil and perfluorocarbon

Background

Tumor recurrence and metastasis occur at a high rate in patients with colon cancer. Identification of effective strategies for the treatment of colon cancer is critical. Recently, poly (lactic-co-glycolic acid) (PLGA) has been shown to have potential as a broad therapeutic drug delivery system. We designed a dual-loaded nanoparticle drug delivery system to overcome the limitations of chemotherapeutic drugs used to treat colon cancer.

Methods

We developed epidermal growth factor (EGF) functionalized PLGA nanoparticles (NPs) co-loaded with 5-fluorouracil (5Fu) and perfluorocarbon (PFC) (EGF-PLGA@5Fu/PFC) for targeted treatment of colon cancer. CCK-8 assay, Hoechst33342 staining and flow cytometry were performed to investigate the functions of EGF-PLGA@5Fu/PFC NPs in SW620 cells. Beside, animal experiment, histological analysis and immunofluorescence staining were adopted to further confirm the role of EGF-PLGA@5Fu/PFC NPs in vivo.

Results

The findings showed that EGF-PLGA@5Fu /PFC NPs had an average size 200 nm and a 5Fu-loading efficiency of 7.29%. Furthermore, in vitro release was pH-sensitive. Targeted EGF-PLGA@5Fu/PFC NPs exhibited higher cellular uptake than non-targeted NPs into colon cancer cells. In addition, EGF-PLGA@5Fu/PFC NPs suppressed cell viability and induced apoptosis in SW620 cells to a greater extent than non-targeted NPs. In tumor xenografted mice, EGF-PLGA@5Fu/PFC NPs suppressed tumor growth more effectively than 5Fu, PLGA@5Fu or PLGA@5Fu/PFC NPs. Histopathological analysis further demonstrated that EGF-targeted NPs inhibited tumor growth to a greater extent than non-targeted or non-NP treatments. The improved therapeutic outcomes observed in this study were due to relief of tumor hypoxia by transport of oxygen by PFC to the tumors.

Conclusion

We constructed a biocompatible nanodrug delivery system based on functionalized nanoparticles that provided a novel strategy for selective delivery of chemotherapy drugs to tumors.

Regioselective synthesis of difluoroalkyl/perfluoroalkyl enones via Pd-catalyzed four-component carbonylative coupling reactions

The first example of Pd-catalyzed four-component carbonylative difluoroalkylation/perfluoroalkylation through the alkyne difunctionalization process is achieved.

Air-stable, heme-like water-soluble iron(II) porphyrin: in situ preparation and characterization

Preparation of the water-soluble, kinetically labile, high-spin iron(II) tetrakis(4-sulfonatophenyl)porphyrin, Fe(II)TPPS(4-), has been realized in neutral or weakly acidic solutions containing acetate buffer. The buffer played a double role in these systems: it was used for both adjusting pH and, via formation of an acetato complex, trapping trace amounts of iron(III) ions, which would convert the iron(II) porphyrins to the corresponding iron(III) species. Fe(II)TPPS(4-) proved to be stable in these solutions even after saturation with air or oxygen. In the absence of acetate ions, however, iron(II) ions play a catalytic role in the formation of iron(III) porphyrins. While the kinetically inert iron(III) porphyrin, Fe(III)TPPS(3-), is a regular one with no emission and photoredox properties, the corresponding iron(II) porphyrin displays photoinduced features which are typical of sitting-atop complexes (redshifted Soret absorption and blueshifted emission and Q absorption bands, photoinduced porphyrin ligand-to-metal charge transfer, LMCT, reaction). In the photolysis of Fe(II)TPPS(4-) the LMCT process is followed by detachment of the reduced metal center and an irreversible ring-opening of the porphyrin ligand, resulting in the degradation of the complex. Possible oxygen-binding ability of Fe(II)TPPS(4-) (as a heme model) has been studied as well. Density functional theory calculations revealed that in solutions with high acetate concentration there is very little chance for iron(II) porpyrin to bind and release O(2), deviating from heme in a hydrophobic microenvironment in hemoglobin. In the presence of an iron(III)-trapping additive that is much less strongly coordinated to the iron(II) center than the acetate ion, Fe(II)TPPS(4-) may function as a heme model.

Assessment of newly developed perfluorocarbon emulsion: oxygen carrying capacity as the blood substitute in vivo

This study provides the evaluation of oxygen carrying capacity of the novel perfluorocarbon emulsion (Neo-PFC) produced by the new emulsifying technology named High Pressure Process. For the performance comparison of oxygen carrying abilities of Neo-PFC and a representative PFC emulsion, the oxidation states of cerebral tissues in substituted animals were measured by near-infrared spectrometry. After the 70% exchange transfusion of whole blood of rats by Neo-PFC and Fluosol-DA, fractional inspired oxygen (FiO2) was gradually decreased from 100% to 0%. As the control experiments, the blood was substituted by Krebs Ringer bicarbonate buffer containing 3% BSA. When the blood of rats was substituted by Neo-PFC, Cyt. ox., a terminal enzyme in mitochondrial respiratory chain maintained fully oxidized state with FiO2 values between 100 to 40%. By contrast, in the models substituted by Fluosol-DA and BSA-buffer. Cyt. ox. was gradually reduced with FiO2 values below 60% and 80%, respectively. This specific advantage of Neo-PFC was explained by its higher oxygen solubility in arterial blood. The novel PFC emulsion prepared by the new emulsifying technology is a potential basis for blood substitutes.

Gas Embolism: Part II. Arterial Gas Embolism and Decompression Sickness

Gas emboli syndromes are known to occur in many different settings, and they may result in life-threatening emergencies. Venous gas embolization was discussed previously in Part I of this review. Gas emboli that gain access to the arterial circulation or that result from exposures to decreased ambient pressures in the environment are discussed in Part II. The prevalence of arterial gas emboli and decompression sickness are likely not as high as for venous gas emboli. Most cases are preventable, and prompt treatment is frequently effective. Once present, gas bubbles generally distribute themselves throughout the body based on the relative blood flow at the time, thus making the nervous system, heart, lung, and skin the primary organ systems involved. Both mechanical and biophysical effects lead to intravascular and extracellular alterations that result in tissue injury. The clinical manifestations of these disorders are varied, and a high index of suspicion in the appropriate settings will aid health care providers in prompt recognition of these problems and allow timely intervention with specific therapy. Management of arterial gas emboli and decompression sickness is similar, with a focus on hyberbaric chamber therapy and intermittent hyperoxygenation. Recompression schedules in current use have withstood the test of time. Research continues to refine our understanding of these diseases and to optimize the treatment regimens available.

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.

Current status of injectable oxygen carriers

In this review the current status of what commonly are termed "blood substitutes" is discussed. The term blood substitute is a misnomer because the formulations under development at this time transport respiratory gases but do not perform the metabolic, regulatory, and protective functions of blood. Either hemoglobin or a perfluorochemical form the base to transport oxygen; the advantages and disadvantages of each base are discussed. The availability of a blood substitute in the U.S. will require approval by the Food and Drug Administration (FDA) and, by law, both its efficacy and safety must be demonstrated prior to approval. Showing efficacy of any blood substitute is complicated by the oxygen reserve and the compensatory mechanisms to acute blood loss in man. The challenge is to prove that the administration of these formulations offer clinical advantages compared with replacement of volume alone. Several efficacy models, the most attractive among them being perioperative hemodilution, should provide data that would bring these formulations into clinical practice. When hemoglobin is not within the favorable environment of the red cell, whether the hemoglobin is derived from expression vectors developed through recombinant biotechnology or from lysed human red cells, it acquires a left-shifted oxygen disassociation curve. Further, because the tetramer disassociates when injected intravenously and the resulting dimers are cleared rapidly from the circulation by the kidneys, intravascular dwell time is brief. Hemoglobins have been modified chemically and linked intramolecularly, intermolecularly, and to macromolecules to correct these problems. While these manipulations have normalized the p50 and extended the dwell time significantly, some toxicity problems remain unresolved. The binding of nitric oxide to hemoglobin preparations and the presumably resultant systemic and pulmonary hypertension observed in animals may be the most difficult to overcome, although the implications of these reactions in man is poorly understood. Perfluorochemicals (PFC) provide a fundamentally different and simpler approach to oxygen transport than hemoglobin formulations. Typically, the PFCs used are liquids composed of 8 to 10 carbon atoms that dissolve oxygen and obey Henry's law. Thus, the recipient's inspired oxygen and cardiac output assume importance. Because they are insoluble in water, PFCs are administered as emulsions, that is, as small droplets about 0.1 to 0.2 microns in diameter. In this respect, they are very similar to the lipid emulsions widely used for parenteral nutrition. Egg yolk phospholipid and poloxamers are most commonly used as emulsifiers. PFCs are not metabolized and are excreted unchanged by the lungs, following temporary storage by the monocyte-macrophage system (MMS).(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of perflubron emulsion particle size on blood half-life and febrile response in rats

Perfluorochemical (PFC) emulsions are particulate in nature and, as such, can cause delayed febrile reactions when injected intravenously. This study investigated the influence of emulsion particle size on intravascular retention and on body temperature changes in unrestrained conscious rats. Concentrated (60% to 90% w/v) emulsions based on perflubron (perfluorooctyl bromide [PFOB]) with mean particle sizes ranging from 0.05 microns to 0.63 microns were tested. Rats were fitted with a chronic jugular catheter and an abdominal body temperature telemetry unit. Fully recovered, conscious rats were monitored for 24 hours after infusion (dose = 2.7 g PFC/kg). Emulsion blood half-life (T1/2) was determined from blood perflubron levels measured by gas chromatography. Emulsions with a particle size of 0.2-0.3 microns caused fevers (6 to 8 hour duration) which peaked at 1-1.5 degrees C above normal (approximately 37.5 degrees C). Fevers could be blocked by i.v. treatment with either cyclooxygenase inhibitors (ibuprofen) or corticosteroids (dexamethasone). Both intensity and duration of the temperature response, quantified by area under the temperature curve, was decreased significantly for emulsions with a particle size < or = 0.12 micron. Blood T1/2 varied inversely with particle size, and was 3 to 4 fold longer for emulsions with a mean particle size < or = 0.2 micron. Thus, smaller emulsion particles more effectively evaded the reticuloendothelial system, which resulted in longer intravascular retention, less macrophage activity, and reduced febrile responses.

Enhanced oxygen delivery by perflubron emulsion during acute hemodilution

A high-concentration 90% w/v perflubron (perfluorooctyl bromide [PFOB]) emulsion (Oxygent HT) is being evaluated as an oxygen carrier for use during surgery. This study was done to assess oxygen delivery by Oxygent HT during acute normovolemic hemodilution. Anesthetized mongrel dogs, instrumented with femoral and pulmonary artery catheters, were hemodiluted to a hematocrit of 25% with 3:1 (v/v) of Ringers-lactate (R-L). Dogs were then ventilated with 100% O2 and hemodiluted to a Hct approximately 11% with 1.5 (v/v) of colloid (autologous plasma and 5% albumin). Dogs then received either 3.3 mL/kg Oxygent HT (n = 5) or 3.3 mL/kg R-L (n = 4), and were monitored for 3 hours. Total oxygen delivery (DO2), blood oxygen content, cardiac output, mixed venous PO2, and mixed venous Hb saturation was higher in Oxygent HT treated dogs compared to the R-L controls. The percentage of total DO2 contributed by perflubron-dissolved oxygen was about 8-10% and accounted for 25-30% of total oxygen consumption (VO2). The percentage of VO2 contributed by Hb-carried oxygen was significantly higher in R-L controls (46 +/- 4%) than in the treated dogs (15 +/- 3%), indicating that the availability of the perflubron-dissolved oxygen allowed for a reserve of oxygen to remain available in the red blood cells.

Emulsified perfluorochemicals as respiratory gas carriers: recovery of perfluorodecalin emulsion droplets from rat tissues

To further understand the in-vivo biokinetic behaviour of perfluorochemical (PFC) emulsions, male rats were injected (10 mL kg-1) with 30% (w/v) emulsified perfluorodecalin (FDC) and uptake into tissues assessed. At 72 h after injection, the mean (+/- s.e.m.) diameters of FDC droplets recovered from liver and spleen were 2.24 +/- 0.04 and 2.78 +/- 0.10 microns, respectively; droplets recovered from lung after 72 h (mean: 1.73 +/- 0.13 micron) were significantly smaller (P < 0.01). After 7 days, FDC droplet diameters in liver had increased to 3.31 +/- 0.13 microns (P < 0.01) and those in lung to 2.71 +/- 0.14 microns (P < 0.01); droplets in spleen after 7 days (2.22 +/- 0.09 microns) were similar to those at 72 h. These data support the hypothesis that significant initial coalescence of FDC droplets occurs in the rat liver and spleen, with further coalescence in the liver up to 7 days. The mean percentage of the injected FDC dose recovered from the liver after 72 h was 2.2 +/- 0.4%, and after 7 days was 0.07 +/- 0.05% (P < 0.01). A smaller decrease in the percent injected FDC in spleen also occurred over the same period (72 h: 1.9 +/- 0.3%; 7 days: 0.8 +/- 0.5%; P < 0.01). The percent injected FDC in lung was similar at 72 h (0.007 +/- 0.004%) and 7 days (0.005 +/- 0.001%). FDC was undetectable (< 0.001%) in all blood samples. The greater rate of FDC elimination from the liver than from the spleen may be related to differences in the rates of reticuloendothelial system processing between these organs.

Uptake of concentrated perfluorocarbon emulsions into rat lymphoid tissues

The effects of injecting (10-30 mL kg-1) either perfluorodecalin (FDC) emulsions of increasing phase fraction (20-60% w/v) or the commercial formulation, Fluosol, on lymphoid tissues have been studied for up to 7 days in male rats. Tissue weights increased by up to 123% (P < 0.05) in proportion with quantity of perfluorochemical (PFC) injected, with spleen responses consistently greater than those of the liver. PFC droplets recovered from these tissues at 72 h after injection of 30% (w/v) FDC emulsion (10 mL kg-1) had mean diameters in the 1-10 microns range, with those from the spleen being larger than those from the liver. Recovered droplet diameters were considerably greater than freshly-prepared emulsion mean particle sizes (0.21-0.25 microns). These results suggest that coalescence of emulsion droplets following accumulation in lymphoid tissue is a pre-requisite for the eventual excretion of PFC vapour through the lungs.

Perioperative haemotherapy: I. Indications for blood component transfusion

The practice of transfusion medicine has undergone substantial change over the last decade. Much of the impetus for the change has come from the isolation of human immunodeficiency virus (HIV) and the linkage of HIV transmission to blood transfusion. The purpose of this paper is to collate and review the literature relating to the indications for blood transfusion and provide recommendations for the appropriate utilization of blood products. Peer-reviewed and published studies and reviews relating to aspects of clinical blood transfusion were identified through computer searches and searching of the bibliographies of identified articles. Emphasis was placed on the literature published within the last decade and particularly in the years 1985-91. Material was chosen which was of proved clinical importance and in which findings were consistent among different investigators or different centres. Less emphasis was placed on material reporting new findings of uncertain clinical relevance or findings that were not consistent with majority reports. It is concluded that the only indication for red cell transfusion is to increase the oxygen carrying capacity of the blood and that an adjustment downwards in the haemoglobin concentration at which blood is transfused (transfusion trigger) from the traditional level of 100 g.L-1 is supported by the physiological and clinical data. Perioperative haemoglobin concentrations of 80 g.L-1 are acceptable in otherwise healthy young patients. The transfusion trigger should be adjusted upwards from this in medically compromised patients and in the elderly (greater than 60 yr). Fresh frozen plasma (FFP) is only indicated when there are documented deficiencies of coagulation factors. Platelet concentrates (PC) are indicated for the treatment of clinical coagulopathy resulting from thrombocytopaenia or platelet dysfunction. Routine or prophylactic administration of either FFP or PC after cardiopulmonary bypass or during resuscitation from haemorrhage is not indicated.

Biocompatibility studies with perfluorochemical oxygen carriers

The effects of injecting either a novel perfluorodecalin (FDC)-based emulsion or various perfluorochemical (PFC) oils on liver cytochromes P-450 (P-450) and aryl esterase (LAE) enzymes in male rats have been studied. Mean P-450 concentration increased (P less than 0.05) following injection of the novel emulsion or FDC. In contrast, LAE increased in response to injection of FDC (P less than 0.05) and the C-16 oil, perfluoroperhydrofluoranthrene (P less than 0.01), added to stabilize the emulsion. No corresponding changes in either P-450 or LAE occurred in rats injected with either perfluorotripropylamine, perfluorotributylamine or perfluorooctylbromide. The "sleeping time" of animals injected with the pentobarbital-based anaesthetic, Equithesin, following pretreatment with the novel emulsion was lower (P less than 0.05) than in saline-injected controls. The results are discussed in relation to the effects of PFCs on liver metabolic functions.

Potential clinical applications for blood substitutes

In the coming decade, it is likely that oxygen-carrying alternatives to red blood cells will become available for clinical use. The driving force behind their development is the risk of transfusion of homologous blood, which includes transmission of viral disease (HIV and hepatitis) and transfusion reactions as well as the expense of collecting and storing human blood. A number of clinical applications for these products can be anticipated now, but when available, it is likely that the list will grow. How widely these products will be used depends on their safety. In addition to these clinical applications, blood substitutes will be useful in furthering our understanding of basic oxygen transport physiology.

Protective effects of a novel perfluorochemical emulsion in photodynamic therapy

The effects of pre-injection of mice with a novel perfluorodecalin-based emulsion on the responses to photodynamic therapy (PDT) using the photosensitizer, metatetra (hydroxyphenyl) porphyrin (m-THPP), have been studied. Injection of emulsion after m-THPP and before illumination (activating wavelength 648 nm) protected skin against PDT-induced inflammatory effects, as reflected by decreases (P less than 0.05) in vascular permeability and oedema formation. However, there was no protection against epidermal cell loss. In contrast, injection of emulsion before sensitizer had no corresponding effect. A fall in mean dermal temperature of up to 6 degrees C occurred in mice injected with emulsion 1-2.5 h before illumination suggesting a decrease in skin blood flow which would reduce oedema formation. Possible mechanism(s) for this apparent protective effect are discussed.

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.