Stability of liposomal formulations: action of amphiphilic molecules

The destabilization of vesicles obtained by cyclic freezing-thawing and extrusion (FATVET) bilayer membranes consisting of L-alpha-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 degrees C (T < T-m) is three times greater than that at 50 degrees C (T > T-m) also, R-t(sol), which corresponds to complete solubilization of the bilayers, is twice higher at 25 degrees C than it is at 50 degrees 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 degrees C and 50 degrees 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 degrees C than it is at 50 degrees C. On the other hand, the R-e(sat), R-e(sol) and K values for FATVETs{egg yolk lecithin (EYL)} at 25 degrees C are virtually identical with those for FATVETs{DPPC} at 50 degrees 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.