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Semifluorinated Alkanes As Stabilizing Agents of Fluorocarbon Emulsions

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Semifluorinated Alkanes As Stabilizing Agents of Fluorocarbon Emulsions Semifluorinated Alkanes as Stabilizing Agents of Fluorocarbon Emulsions Sabina Marie Bertilla, Pascal Marie, and Marie Pierre Krafft Summary. We obtained highly stable, small-sized, and narrowly dispersed perfluorooctyl bromide (PFOB) emulsions using combinations of phospho- lipids and semifluorinated alkanes CnF2n+1CmH2m+1 (FnHm diblocks) as the emulsifying system. For example, after 6 months at 25°C the average droplet diameter of an emulsion stabilized by F6H10 was only ~80 nm, compared to ~180 nm for the reference emulsion stabilized by phospholipids alone. In parallel, a co-surfactant effect has been demonstrated for F8H16 at the water–PFOB interface using pendant drop interfacial activity measurements, which supports the hypothesis that a fraction of the diblocks is located at the phospholipid interfacial film.We also established that the length of the hydro- carbon segment, Hm, must be comparable to that of the phospholipid’s fatty chains to obtain the stabilization effect; a strong mismatch leads to rapid coa- lescence of droplets in the emulsion. For fluorocarbon emulsions, small droplet sizes translate into prolonged intravascular persistence and reduced side effects. FnHm diblocks thus provide a useful, versatile tool to improve the characteristics and stability of injectable fluorocarbon emulsions. Key words. Fluorocarbon emulsion stabilization, Fluorocarbon/hydrocarbon diblock, Ostwald ripening, Molecular diffusion, Interfacial tension Introduction Exceptional biological inertness and gas-dissolving properties are the basis for the development of injectable fluorocarbon (FC)-in-water emulsions as temporary oxygen carriers for use during surgery [1]. Such synthetic “blood Colloïdes et Interfaces, Institut Charles Sadron (CNRS), 6 rue Boussingault, 67083 Strasbourg Cedex, France 237 238 S.M. Bertilla et al. substitutes” are devoid of the inherent complexity of hemoglobin solutions. Perfluorooctyl bromide (C8F17Br) (PFOB), emulsified with egg yolk phospho- lipids, has been investigated in Phase III clinical trials in Europe and the United States [2,3]. Further desirable progress in terms of emulsion charac- teristics includes improvement of the stability of the FC emulsion and reduc- tion of droplet size to prolong intravascular persistence, reduce side effects, and facilitate oxygen diffusion. Our strategy with respect to achieving such improvements consists in reinforcing the interfacial film of the emulsion using nonpolar fluorophilic amphiphiles such as semifluorinated alkanes, CnF2n+1CmH2m+1 (FnHm diblocks). Molecular diffusion (Ostwald ripening) has been determined to be the primary mechanism of coarsening in both dilute and concentrated FC-in- water emulsions over time [4–10]. An effective method for stabilizing PFOB emulsions against molecular diffusion is to add small amounts of perfluo- rodecyl bromide (C10F21Br, PFDB) to PFOB, which reduces the solubility and dispersibility of the dispersed FC phase, thereby reducing molecular diffusion [11]. An alternative means of stabilizing FC emulsions consists in the incor- poration of FnHm diblocks in the phospholipid. Highly effective stabilization of egg yolk phospholipid (EYP)-based concentrated fluorocarbon emulsions has been obtained [12,13]. In the concept that underlies this procedure, the FnHm diblock molecules were expected to be located at the interface between the FC phase and the surrounding phospholipid film (Fig. 1). Whether the observed FC emulsion stabilization by FnHm diblocks is due to an interfacial effect (which supposes that the diblocks are, at least in part, located at the Fig. 1. Surface of a droplet of fluorocarbon (FC)-in- water emulsion stabilized by (a) a phospholipid (PL) alone or (b) a combination of PL and a semifluorinated diblock (FnHm). Owing to the amphiphilic character of the latter, it was hypothesized that the semifluorinated diblock would collect in part at the interface Fluorocarbon Emulsion Stabilization 239 interface) or solely to a reduction in solubility of the dispersed phase (as if the diblock molecules were dispersed uniformly within the FC droplets) has been a matter of debate [9,10,14]. Where biological aspects are concerned, some attention has been given to FnHm diblocks because of the multiple functions they can fulfill. In addition to their use for stabilization and precise control of particle size of FC-in-water emulsions, their use as a “surfactant” allows the preparation of hydrogenated oil-in-FC emulsions [15]. Incorporation of FnHm diblocks in the bilayer of vesicles can increase the vesicles’ shelf stability, reduce their permeability [16,17], slow the kinetics of Ca2+-induced fusion of dipalmitoylphos- phatidylserine small unilamellar vesicles [18], and modify their behavior in a biological medium [19]. FnHm diblocks were demonstrated to form an inter- nal, Teflon-like, fluorinated film within the bilayer of phospholipid vesicles [20]. The synthesis and purification of FnHm is straightforward [21,22]. Pre- liminary studies have indicated that diblocks do not perturb growth or via- bility of cell cultures [13]. No inhibition of carcinoma cell proliferation was noted with C6F13C10H21 (F6H10) [23]. The growth and survival of mice were not affected by the intraperitoneal administration of large doses (30 g/kg body weight) of C6F13CH== CHC10H21 (F6H10E) or F6H10 [13,24]. Even the iodinated mixed compound C6F13CH== CIC6H13 was tolerated intraperitoneally by mice at a dose of 45 g/kg body weight [25]. The half-life of F6H10E was evaluated to be 25 ± 5 days in the liver, the organ that had taken up most (70%) of the injected dose (3.6 g/kg) adminis- tered to rats in the form of a 25% w/v emulsion of pure diblock [13,24]. In the current study, we found that unusually fine, stable PFOB emulsions stabilized by combinations of natural phospholipids (PLs) and FnHm diblocks can be obtained. We also report a set of experiments designed to determine whether semifluorinated diblock molecules interact at the FC–water interface and elucidate the mechanism through which they stabi- lize FC emulsions.The interfacial tension between phospholipid solutions and PFOB containing various concentrations of F8H16 was measured using the pendant drop method. The particle growth in emulsions stabilized by phos- pholipid/diblock combinations was monitored using centrifugal photosedi- mentation or quasielastic light scattering. Experimental Materials The diblocks F6H10 and F8H16 were synthesized according to Brace [21]. They were thoroughly purified by distillation (F6H10) or successive recrys- tallization from chloroform (F8H16). Their purity (>99%) was determined 240 S.M. Bertilla et al. using gas chromatography, thin-layer chromatography, nuclear magnetic res- onance, and elemental analysis. PFOB and PFDB were gifts from Alliance Pharmaceutical Corp. (San Diego, CA, USA). Perfluorohexadecane (PFHD) came from Fluorochem (Ugarit Chimie, Issy les Moulineaux, France), and EYP was kindly donated by Lipoid (Ludwigshafen, Germany). Dioctanoylphos- phatidylcholine (PLC8), dilaurylphosphatidylcholine (DLPC), and dimyris- toylphosphatidylcholine (DMPC) were purchased from Sigma (99% pure). Water was obtained from a MilliQ system (Millipore, Bedford, MA, USA). Methods Emulsion Preparation and Particle Size Measurement A series of emulsions based on EYP, equimolar combinations of EYP and F6H10, and equimolar combinations of EYP and PFHD (C16F34) were prepared by high-pressure homogenization. These emulsions all contained 60% w/v PFOB and 3% w/v phospholipids. Their average droplet diameter and size dis- tribution have been measured after preparation and monitored as a function of time using quasielastic light spectroscopy (Malvern Zeta Sizer 3000 HS; Malvern, Worcestershire, UK). EYP-based Emulsions EYP (3 g) was dispersed in water (100 ml) and homogenized with a low- energy mixer (Ultra-Turrax; Ika-Labortecknik, Staufen, Germany) for 15 min at 13 000 rpm at room temperature. PFOB (60 g) was added dropwise at 9000 rpm. The dispersion was stirred for 30 min at 13 000 rpm in a ther- moregulated bath (5°–10°C). It was further homogenized under high pressure (1000 bars, 30 min) using a Microfluidizer (model 110; Microfluidics, Newton, MA, USA). EYP/additive-based Emulsions EYP (3 g) was dispersed in water (100 ml) and homogenized with a low-energy mixer for 5 min at 13 000 rpm at room temperature. An equimolar amount of the additive (F6H10 or PFHD) was then added. The dispersion was further homogenized at 13 000 rpm for 5 min in a thermoregulated bath (5°–10°C). PFOB addition (60 g), preemulsification, and high-pressure homogenization were achieved as described above. All the EYP-based emulsions were bottled in 10-ml vials. They were then heat-sterilized at 121°C for 15 min and stored at 25°C. Other series of emulsions based on DMPC or PLC8, equimolar combina- tions of each of these phospholipids with F8H16, or equimolar combinations Fluorocarbon Emulsion Stabilization 241 of these phospholipids with PFDB were prepared by high-pressure homoge- nization. Their average droplet diameter and size distribution were measured after preparation and monitored as a function of time using centrifugal pho- tosedimentation (Horiba Capa 700; Horiba, Kyoto, Japan). The emulsions all contained PFOB (30% w/v) and phospholipids (4% w/v). Reference Emulsions DMPC (or PLC8) (1.2 g) was dispersed in water (25 ml) and homogenized with the Ultra-Turrax mixer for 15 min at 13 000 rpm at room temperature. PFOB (9 g) was added dropwise at 9000 rpm. The dispersion was stirred for 30 min at 13 000 rpm
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