Jean G. Riess

Fluorous micro- and nanophases with a biomedical perspective

Abstract Fluorinated components useful for organizing space at the molecular, nanometer and micrometer scales include perfluorocarbons, perfluoroalkylated surfactants and perfluoroalkyl/alkyl diblock amphiphiles. Perfluoroalkyl moieties, being lipophobic as well as highly hydrophobic, add a new dimension to the hydrophobic segregation effect. Fluorinated amphiphiles, therefore, have an enhanced tendency to self-assemble in various media into stable, highly organized fluorinated colloids, thus generating organized nanometer-size fluorous phases, i.e. fluorous domains with at least one dimension in the nanometer range. Such fluorous nanophases are found in variously shaped micelles, Langmuir films, and bilayer membranes, as in vesicles, tubules and other molecular self-assemblies. Micron-size fluorous phases are present in diverse colloids that comprise liquid, solid and gaseous perfluorocarbons, such as in emulsions, microemulsions, multiple emulsions, microbubbles, gels and dispersions. Continuous or dispersed fluorous, hydrocarbonous and aqueous phases can be present simultaneously. Research on colloidal systems involving highly fluorinated components and destined for biomedical uses (injectable O2 carriers, contrast agents, drug delivery systems and other devices) has generated a wealth of data. These data are analyzed here from the perspective of the formation, structure and behavior of fluorous nano- and microphases in colloidal systems. Fluorocarbons and fluorinated amphiphiles allow the formulation of an array of multicomponent, multiphase compartmented colloidal systems and nano-objects with various architectures, differential solubility and diffusibility characteristics, and other properties, and exclusion zones that have potential as microreservoirs, microreactors and templates useful for reaction, morphology and functionality control well beyond their initial biomedical purpose.

Fluorinated materials for in vivo oxygen transport (blood substitutes), diagnosis and drug delivery

Fluorocarbons are characterized by exceptional chemical and biological inertness, extreme hydrophobicity, lipophobicity, high gas-dissolving capacities, low surface tensions, high fluidity and spreading coefficients, high density, absence of protons, and magnetic susceptibilities comparable to that of water. These unique properties are the foundation for a range of biomedical applications. An injectable fluorocarbon-in-water emulsion is in advanced clinical trials as a temporary oxygen carrier (blood substitute) to prevent tissue hypoxia or ischemia in the surgical and critical care patient. A liquid fluorocarbon is in Phase II/III clinical trials for treatment of acute respiratory failure through liquid ventilation. Several fluorocarbon-based contrast agents for ultra-sound imaging are in various stages of clinical investigation. Multiple families of well-defined pure fluorinated surfactants have recently been synthesized. These surfactants have a modular structure which allows stepwise adjustment of their physicochemical characteristics. Their polar head group derives from polyols, sugars, aminoacids, amides, amine oxides, phosphocholine, phosphatidylcholine, etc. Fluorinated surfactants are significantly more surface-active than their hydrocarbon analogs and they display a greater tendency to self-assemble, thus forming well-ordered, stable supramolecular assemblies such as vesicles, tubules, fibers, ribbons, etc. Fluorinated amphiphiles also allowed the obtaining of a variety of stable reverse and multiple emulsions and gels. These systems are being investigated as drug delivery devices.

Fluorocarbon‐Based in vivo Oxygen Transport and Delivery Systems

Abstract. 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.

Perfluorocarbon-based Oxygen Delivery

The basic properties of perfluorocarbons (PFCs) and PFC emulsions relevant to their use as oxygen delivery systems are briefly reviewed. The key issues related to the selection of an appropriate, readily excretable PFC and the engineering of a stable injectable PFC emulsion are discussed. OxygentTM, a terminally heat-sterilized, injectable 60% w/v PFC emulsion made primarily of F-octyl bromide and a few percent of F-decyl bromide, with egg phospholipids as an emulsifier, has been developed. Its efficacy in avoiding and reducing red cell transfusion during surgery has been established during a Phase III clinical evaluation. Another Phase III clinical trial in cardiopulmonary bypass surgery, with a protocol that included both augmented-acute normovolemic hemodilution and intraoperative autologous donation, has, however, been interrupted following the observation of adverse events. Data analysis assigned these events to an inappropriate study protocol. A search for possible interactions between Oxygent and fluids present during cardiopulmonary bypass surgery detected no effect of the emulsion on hemostasis, hemolysis and blood rheology.

Understanding the Fundamentals of Perfluorocarbons and Perfluorocarbon Emulsions Relevant to In Vivo Oxygen Delivery

The unique behavior of perfluorocarbons (PFCs), including their high oxygen dissolving capacity, hydrophobic and lipophobic character, and extreme inertness, derive directly, in a predictable manner, from the electronic structure and spatial requirements of the fluorine atom. Their low water solubility is key to the prolonged in vivo persistence of the now commercially available injectable microbubbles that serve as contrast agents for diagnostic ultrasound imaging. OxygentTM, a stable, small-sized emulsion of a slightly lipophilic, rapidly excreted PFC, perfluorooctyl bromide (perflubron), has been engineered. Significant oxygen delivery has been established in animal models and through Phase II and III human clinical trials. However, an inappropriate testing protocol and the lack of funding led to temporary suspension of the trials.

Extended in vivo blood circulation time of fluorinated liposomes

The clearance from blood circulation of fluorinated liposomes made with perfluoroalkylated phosphatidylcholines was investigated in mice using liposome‐entrapped 5(6)‐carboxyfluorescein. The presence of a fluorinated core inside the membrane strongly retards their blood clearance. The fluorinated vesicles showed circulation half‐lives of up to 8.6 h, which are 6–13 and 3–6 times larger than those of similarly sized conventional distearoylphosphatidylcholine and distearoylphosphatidylcholine/cholesterol liposomes, respectively. Their blood clearance was similar to that of some polyethylene glycol (PEG)‐labelled ‘stealth’ liposomes and was dose‐independent in a 3.3–330 body weight dose range.

Emulsions and microemulsions with a fluorocarbon phase.

Abstract A phase III clinical study of a perfluorooctyl bromide emulsion demonstrated reduction and avoidance of donor blood transfusion in surgery. Novel fluorocarbon-in-water emulsions are being investigated, including emulsions highly stabilized by fluorocarbon–hydrocarbon diblocks and targeted emulsions for molecular imaging, diagnosis and drug delivery. Reverse water-in-fluorocarbon emulsions and microemulsions that have potential for pulmonary drug delivery are also being studied. Microemulsions with highly fluorinated components are being actively investigated, with applications in polymerization technology and as research tools.

Highly fluorinated systems for oxygen transport, diagnosis and drug delivery

Abstract This review summarizes the basic physical and chemical characteristics of fluorocarbons and fluorinated materials (especially surfactants), in neat or dispersed form (emulsions, vesicles), which determine their potential in medicine. These properties are discussed in relation to each major area of application (oxygen delivery, liquid ventilation, diagnosis, drug delivery). Specific topics are emphasized including, in particular, the lipophilic character of certain fluorocarbons, the stability of emulsions, and the stability, permeability and intravascular persistence of fluorinated vesicles.

Blood substitutes and other potential biomedical applications of fluorinated colloids

An oxygen carrying, heat-sterilized phospholipid-based emulsion of a fast excreted lipophilic fluorocarbon, perfluorooctyl bromide (perflubron), stabilized against molecular diffusion, has been developed to serve as a temporary blood substitute. It is expected to reduce exposure to donor blood and thereby help mitigate the pressure on our blood supply. A phase III clinical trial in Europe has demonstrated that use of the emulsion resulted in avoidance and reduction of donor blood transfusion in surgery patients. Further potential applications for fluorocarbon emulsions include use as a bridge to transfusion, treatment of myocardial ischemia and stroke, potentiation of radio and chemotherapy and preservation of organs destined for transplantation. Echogenic, injectable gaseous microbubbles, osmotically stabilized by perfluorohexane, provide an effective contrast agent for ultrasound imaging. Clinical trials have established improved imaging of the walls of the heart and, hence, assessment of cardiac function. The ability of improving detection of myocardial perfusion, blood flow abnormalities and solid tumors is also being investigated. Various families of fluorinated amphiphiles with modular molecular structures and polar heads derived from natural products have been synthesized. Due to their highly hydrophobic perfluoroalkylated tail chains, these amphiphiles readily self-assemble into stable fluorinated vesicles, tubules and other organized molecular systems with distinctive properties. Fluorosurfactants also allowed preparation of direct, reverse, apolar and multiple emulsions and gels. These fluorinated colloids have potential for the delivery of drugs and other bioactive materials, and provide unique tools in biomedical research.

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 bodys 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.