Interaction between liposomes and neutral polymers: Effect of adsorption on drug release

The processes of adsorption of two neutral polymers (poly(vinyl pyrrolidone ), PVP and poly(vinyl alcohol), PVA) were investigated on liposomes compose d of soy lecithin/dicetyl phosphate/cholesterol = 25:2:3 (molar ratio). The liposomes were prepared in buffered solution at pH = 7.4 and mixed with th e solution of the measured polymers in the desired polymer/lipid (w/w) rati os. Adsorption was measured by, determination of the equilibrium bulk conce ntration of the polymer. In the case of PVA quantitative adsorption measure ments with a specific reagent were possible. Adsorption isotherms were reco rded at 25 +/- 1 degreesC It was concluded that adsorbed and unadsorbed PVA molecules are in equilibrium even at low polymer/lipid ratios. The results were confirmed by dynamic laser light scattering (DLS), and thermal activi ty monitoring (TAM) experiments. Another group of the liposomes was prepare d in 60 mM ammonium sulphate (pH= 5.0) and we filled the vesicles with a te st dye, acridine orange (AO) using the pH-gradient (remote loading) method. The AO release property of liposomes was tested with a special vertical di ffusion cell after we had made PVA adsorb on their surface in different PTA /lipid (w/w) ratios.