Catanionic Vesicle-PEG-Lipid System: Langmuir Film and Phase Diagram Study Amir Berman,* ,†,‡ Meir Cohen, † and Oren Regev* ,‡,§ Departments of Biotechnology Engineering-Institute for Applied Biosciences and Chemical Engineering and Ilse Katz Center for Nanoscale Science and Technology, Ben-Gurion University of the Negev, 84105 Beer-Sheva, Israel Received November 2, 2001. In Final Form: May 7, 2002 Catanionic surfactant systems containing PEG-lipid molecules are studied at the air-solution interface and in bulk. It is found that, upon introduction of the optimum amount of PEG-lipid molecules, the region of vesicle stability in the bulk is increased. At the air-solution interface, the system forms an equimolar (salt) film upon compression. The transition to the salt structure in the presence of PEG-lipid took place at higher surface pressure, thus supporting the results of increased stability observed with the bulk samples. The presence of PEG-lipid molecules induces film buckling, resulting in significantly smaller areas per molecule. The combined results are discussed in terms of electrostatic and steric repulsion forces. 1. Introduction Novel means of drug delivery, including slow drug release, targeting, gene therapy, and others, are based on encapsulation of the active ingredients in vesicles. 1-3 The vesicle stability in the body environment and its life span are prime parameters for the effectiveness of such treatments. Phospholipid vesicles doped with PEG-bearing lipids (termed PEG-lipids or lipopolymers) have been demonstrated to evade the immune system for long times; thus, they have been dubbed “stealth liposomes”. 4,5 Stealth liposomes have attracted considerable attention as po- tential drug delivery agents because of their low or inert immune response, originating from their PEG coating, which makes them almost indistinguishable from the body aqueous environment. Although these liposomes are already in routine and exploratory clinical use, various aspects of their stability and performance are still under investigation. These liposomes can be prepared with various lipid compositions, PEG lengths, and PEG surface densities. The presence of PEG chains on the vesicle surface resembles grafted polymers on a rigid surface on one hand, while cooperatively affecting the phase behavior of the lipid-based system, on the other. The performance of mixed-surfactant systems is often superior to that of single-component surfactant systems. Hence, in industrial applications surfactant mixtures are almost always used. Catanionic mixtures are aqueous mixtures of oppositely charged surfactants that display an ability to spontaneously form stable vesicles at high dilution. 6-8 In this study, we explore the effect of the PEG-lipid molecules on the catanionic system in the bulk and at the air-solution interface as a model system for stealth drug delivery. A possible advantage of using such a model system is its sensitive phase response to changes in various conditions such as concentration, composition, and ionic strength, which is manifested by complex phase diagrams (Figure 1). In the bulk, we studied the effect of the addition of PEG- lipid molecules on the phase behavior and the stability of the vesicular region. At the air-solution interface, we formed monolayers of an analogous system and observed the changes in compression isotherms upon partial substitution of the surfactants by PEG-lipid molecules at different cationic-to-anionic molar ratios. 2. A Model System Sodium dodecyl sulfate (SDS), a single-chain anionic surfactant, and didodecyldimethylammonium bromide (DDAB), a double-chained cationic surfactant, were used in this study. The PEG-lipid DPPE-PEG 2000 (where DPPE is dipalmitoylphosphatidylethanolamine) was added to the catanionic system at various molar ratios. The phase behavior of the catanionic mixture of SDS and DDAB in water has been studied in detail. 9-11 Two lobes of isotropic vesicular phases exist near the water apex in the triangular phase diagram (Figure 1A). The upper and lower lobes with excess DDAB and SDS, 9 correspond to regions of positive and negative net charge, respectively. We chose to study the relatively larger area of the SDS-rich lobe, where polydispersed small vesicles are found (Figure 1A). 9 The phase diagram for the PEG-lipid-free bulk system depicts the domains in which stable vesicles are present (Figure 1A). In particular, the lobe marked V (also known as L4 12 ) is made up exclusively of vesicles. In the present * E-mails for correspondence: aberman@bgumail.bgu.ac.il and oregev@bgumail.bgu.ac.il. † Department of Biotechnology Engineering-Institute for Applied Bioscience. ‡ Ilse Katz Center for Nanoscale Science and Technology. § Department of Chemical Engineering. (1) Langer, R. AIChE J. 2000, 46 (7), 1286. (2) Barenholz, Y. Curr. Opin. Colloid Interface Sci. 2001, 6 (1), 66. (3) Cohen, M. The effect of attached PEG substitution on aggregation forms of surfactants. M.Sc. Thesis, Ben-Gurion University of the Negev, Beer-Sheva, Israel, 1998. (4) Allen, T. M.; Hansen, C.; Rutledge, J. Biochim. Biophys. Acta 1989, 981 (1), 27. (5) Allen, T. M. Trends Pharm. Sci. 1994, 15, 215. (6) Herrington, K. L.; Kaler, E. W.; Miller, D. M.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (7) Kaler, E. W.; Herrington, K. L.; Kamalakara, M.; Zasadzinski, J. A. J. Phys. Chem. 1992, 96, 6698. (8) Khan, A.; Marques, E. Catanionic Surfactants; Robb, I. D., Ed.; Blackie Academic and Professional: London, 1997; p 37. (9) Marques, E.; Regev, O.; Khan, A.; Lindman, B.; Miguel, M. J. Phys. Chem. B 1998, 102, 6746. (10) Marques, E.; Regev, O.; Khan, A.; Miguel, M.; Lindman, B. J. Phys. Chem. B 1999, 103, 8353. (11) Regev, O.; Marques, E. F.; Khan, A. Langmuir 1999, 15, 642. (12) Regev, O.; Guillemet, F. Langmuir 1999, 15, 4357. 5681 Langmuir 2002, 18, 5681-5686 10.1021/la011633+ CCC: $22.00 © 2002 American Chemical Society Published on Web 06/26/2002