Stability of liposomal formulations: action of amphiphilic molecules

Abstract The destabilization of vesicles obtained by cyclic freezing-thawing and extrusion (FATVET) bilayer membranes consisting of L- α -dipalmitoyl phosphatidylcholine (DPPC) by the bile salt sodium deoxycholate (DOC) was studied from turbidimetric, entrapped solute release and steady-state fluorescence anisotropy measurements. Based on turbidity measurements, bilayer membranes in a rigid gel state are more resistant to lysis by deoxycholate than are bilayers in a fluid liquid-crystal state. Thus, the [detergent] total /[phospholipid] mole ratio, R t sat , needed to saturate the bilayers at 25°C ( T T m ) is three times greater than that at 50°C ( T > T m ) also, R t sol , which corresponds to complete solubilization of the bilayers, is twice higher at 25°C than it is at 50°C. However, this does not mean that bilayer saturation or complete solubilization calls for increased DOC concentrations, as shown by the fact that the effective mole ratios for both events, R e sat and R e sol , are identical at 25°C and 50°C. The ability of DOC molecules to intercalate themselves between DPPC molecules therefore appears to be decreased by tightly packed phospholipid molecules. Thus, the partition coefficient for the sub-solubilization range, R e , is almost 3 times lower at 25°C than it is at 50°C. On the other hand, the R e sat , R e sol and K values for FATVETs{egg yolk lecithin (EYL)} at 25°C are virtually identical with those for FATVETs{DPPC} at 50°C. Hence, for a series of phospholipid analogues, the partition coefficient K for a surfactant/liposome system is bound to depend primarily on the physical state of the bilayer. The retention properties of FATVETs{DPPC} are impaired at sub-lytic DOC levels thus, 5-fluorouracil (5-FU) leakage under these conditions amounts to R e 50% =0.09, which is somewhat smaller than R e sat (1.2–1.4). This may be the result of the packing and structural changes undergone by lamellae prior to solubilization of the bilayers. The 1,6-diphenyl-1,3,5-hexatriene (DPH) steady-state fluorescence anisotropy results also suggest that DOC alters acyl chain packing, diminishes transition cooperativeness and decreases the orderliness of acyl chains.