Interaction of Polyethylenglycol Coated Liposomes and Plain Liposomes Studied using Fluorescent Probes D. Polo 1 , I. Haro 1 , M. A. Alsina† 2 and F. Reig 1 1 Peptides Department, CID–CSIC, Jordi Girona 18, 08034 Barcelona, Spain 2 Physicochemical Unit, Faculty of Pharmacy, University of Barcelona, Plaza Pius XII, 08028 Barcelona, Spain Biomed. Chromatogr. 11, 75–76, (1997) No. of Figures: 2. No. of Tables: 0. No. of Refs: 4. INTRODUCTION Liposomes have been used extensively as carriers for drug administration and targeting. Their in vivo half life can be dramatically increased by coating the surface with poly- ethylene glycol (PEG) chains (Trubetskoy and Torchilin, 1995). Although phosphatidyl ethanolamine–PEG derivative (PG–PEG) only represents around 4% of total lipids, the length and hydrophilicity of PEG chains can modify to some extent the stability of liposomes as far as leakage, microviscosity of bilayers etc. are concerned. In the present paper we compare some physicochemical characteristics of four liposomal preparations currently used in biological studies. EXPERIMENTAL Chemicals. Phosphatidyl choline (PC) was a preparation of hydrogenated egg lecithin from Asahi (Japan). Phosphatidyl glycerol (PG) and cholesterol (Ch) were from Sigma. Doxorubicin (DX) was obtained from Farmitalia, Carlo Erba (Italy). Diphenyl hexatriene (DPH) was supplied by Fluka. Di-stearoyl phosphatidyl ethanolamine–PEG (DSPE–PEG) of 2000 Da molecular weight was from Shearwater Polymers Europe (Netherlands). Peptide E(8) (17 AA) and N-glutaryl phosphatidyl ethanolamine (NGPE) were synthesized in our laboratory. Details of the syntheses and physicochemical studies are described elsewhere (Polo et al., 1996). Physicochemical studies. Surface activity was determined using a Langmuir-Blodgett balance working with a mini- cuvette of Teflon (70 mL capacity). Compression iostherms were carried out spreading liposomal suspensions on a subphase of sodium chloride 0.9%. Bilayer fluidity was studied through polarization changes of DPH molecules, inserted in lipid bilayers, in the temperature range of 25–60°C. Details of all these procedures are given in Bogdam et al. (1994). Liposome preparation. Vesicles were prepared mixing all the lipidic components in an organic medium, followed by evapora- tion, rehydration, probe sonication and incubation with doxorubicin. When necessary the peptide was linked to the vesicle surface through the carboxyl moiety of NGPE. RESULTS Liposomes. Liposomes were prepared according to standard procedures given in the literature (Nagayasu et al., 1994). Final preparations were characterized by size and phos- pholipid, peptide and doxorubicin quantification. Vesicle compositions were: (A) PC/PG/Ch/DX; (B) PC/PG/Ch/DX/ PE–PEG; (C) PC/PG/Ch/DX/PE–PEG/NGPE/E(8); (D) PC/PG/Ch/PE–PEG/NGPE/E(8).  Correspondence to: F. Reig. Figure 1. Time course of surface pressure increases determined for C liposomes. Figure 2. Surface pressure increases versus concentration for B, C and D liposomes. CCC 0269–3879/97/020075–02 $17.50 Received 21 May 1996 © 1997 by John Wiley & Sons, Ltd. Accepted 17 June 1996 BIOMEDICAL CHROMATOGRAPHY, VOL. 11, 75–76 (1997)