Detailed Structure of Hairy Mixed Micelles Formed by Phosphatidylcholine and PEGylated Phospholipids in Aqueous Media Lise Arleth,* , Beena Ashok, ‡ Hayat Onyuksel, ‡ Pappannan Thiyagarajan, § Jaby Jacob, §,| and Rex P. Hjelm* , Manuel Lujan Jr. Neutron Scattering Center, MS H805, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Biopharmaceutical Sciences, College of Pharmacy, University of Illinois at Chicago, 833 South Wood Street, Chicago, Illinois 60612, and Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received September 29, 2004. In Final Form: January 17, 2005 Aqueous dispersions of mixed egg yolk phosphatidylcholine (PC) and poly(ethylene glycol) (PEG) modified distearoyl phosphatidylethanolamine (DSPE) were investigated with the purpose of determining shape, size, and conformation of the formed mixed micelles. The samples were prepared at a range of DSPEPEG to PC molar ratios ([DSPEPEG/PC] from 100:0 to 30:70) and with, respectively, DSPEPEG2000 and DSPEPEG5000, where 2000 and 5000 refer to the molar masses of the PEG chains. Particle shape and internal structure were studied using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS). The contrast of the micelles is different for X-rays and neutrons, and by combining SANS and SAXS, complementary information about the micelle structure was obtained. The detailed structure of the micelles was determined in a self-consistent way by fitting a model for the micelles to SANS and SAXS data simultaneously. In general, a model for the micelles with a hydrophobic core, surrounded by a dense hydrophilic layer that is again surrounded by a corona of PEG chains in the form of Gaussian random coils attached to the outer surface, is in good agreement with the scattering data. At high DSPEPEG contents, nearly spherical micelles are formed. As the PC content increases the micelles elongate, and at a DSPEPEG/PC ratio of 30:70, rodlike micelles longer than 1000 Å are formed. We demonstrate that by mixing DSPEPEG and PC a considerable latitude in controlling the particle shape is obtained. Our results indicate that the PEG chains in the corona are in a relatively unperturbed Gaussian random coil conformation even though the chains are far above the coil-coil overlap concentration and, therefore, interpenetrating. This observation in combination with the observed growth behavior questions that the “mushroom-brush” transition is the single dominating factor for determining the particle shape as assumed in previous theoretical work (Hristova, K.; Needham, D. Macromolecules 1995, 28, 991-1002). I. Introduction Hairy micelles formed by poly(ethylene glycol) (PEG) modified phospholipid or block copolymers containing PEG have recently attracted a great deal of interest due to the applications as lipid assisted drug-delivery systems. 2-7 The hydrophobic core of the micelles allows for solubili- zation of hydrophobic drugs. The extremely low critical micellar concentration of these systems make the drug- loaded micelles stable upon the fast dilution that takes place when the drug is injected in the blood vessels. In addition, the PEG corona that forms around the micelles provides a shield against attacks from naturally occurring enzymes, which would otherwise quickly destabilize the micelles. The PEGylated micelles can be regarded as the micellar counterpart to the PEGylated vesicles known as “stealth liposomes” which are already used in several medical products. Because of the large pharmaceutical interest in the “stealth liposomes” most of the published research on mixed phospholipid/PEGylated phospholipid systems has been conducted in the part of the phase diagram where the PEGylated vesicles form. The studies have been less focused on the series of morphological changes that the micelles undergo as a function of PEGylated phospholipid/ phospholipid mixing ratio. However, the theoretical work of Hristova and Need- ham 1 and the experimental cryo-transmission electron microscopy (cryo-TEM) work of Edwards et al. from 1997 8 showed that a series of morphological transformations from spherical micelles, 1,8,9 through rodlike micelles, 1,8 disk-shaped aggregates, 8 bilayer vesicles, 1,8 and planar bilayers/lamellar structures 1 will take place as the PEGylated phospholipid/phospholipid molar ratio de- creases. The order of the appearance of the different aggregate morphologies is qualitatively in agreement with the predictions from the critical packing parameter theory * Corresponding authors. E-mail: lia@kvl.dk (L.A.); hjelm@ lanl.gov (R.P.H.). Present address (L.A.): Department of Natural Sciences, Royal Veterinary and Agricultural Unviersity, Bu¨ lowsvej 17, 1870 Frederiksberg, Denmark. Los Alamos National Laboratory. ‡ University of Illinois at Chicago. § Argonne National Laboratory. | Present address: Amgen, Inc., One Amgen Center Drive, Thousand Oaks, CA 91320-1799. (1) Hristova, K.; Needham, D.; Macromolecules 1995, 28, 991-1002. (2) Otsuka, H., Nagasaki, Y.; Kazunori, K. Adv. Drug Delivery Rev. 2003, 55, 403-419. (3) Lin, W.-H., Juang, L.-W.; Lin, C.-C. Pharm. Res. 2003, 20 (4), 668-673. (4) Krishnadas, A.; Rubinstein, I.; O¨ nyu¨ ksel, H. Pharm. Res. 2003, 20 (2), 297-302. (5) Burt, H. M.; Zhang, X.; Toleikis, P.; Embree, L.; Hunter, W. L. Colloids Surf.,B 1999, 16, 161-171. (6) Riley, T.; Govender, T.; Stonik, S.; Xiong, C. D.; Garnett, M. C.; Illum, L.; Davis, S. S. Colloids Surf., B 1999, 16, 147-159. (7) Torchilin, V. P. J. Controlled Release 2001, 73, 137-172. (8) Edwards, K.; Johnsson, M.; Karlsson, G.; Silvander, M. Biophys. J. 1997, 73, 258-266. (9) Needham, D.; McIntosh, T.; Lasic, D. Biochim. Biophys. Acta 1992, 40, 1108. 3279 Langmuir 2005, 21, 3279-3290 10.1021/la047588y CCC: $30.25 © 2005 American Chemical Society Published on Web 03/17/2005