Use of the gamma-ray perturbed angular correlation (PAC) technique for monitoring liposomal phospholipid bilayer integrity.

A membrane labeling method based on the principle of gamma-ray perturbed angular correction (PAC) was developed to monitor the structural integrity of liposomal membranes. The reporter group was 111In(III) complexed with the lipophilic diethylenetriaminepentaacetic acid (DTPA) derivative of dipalmitoylphosphatidylethanolamine (DPPE) embedded in the phospholipid bilayers of small unilamellar liposomes. Using this method, complete chemical digestion of the constituent phospholipids in these DTPA-conjugated liposomes by phospholipase A2 or phospholipase C in the presence of Ca2+ was found not to be followed by an immediate disruption of the liposomal membrane. Compared with other methods, the method developed permits the continuous noninvasive monitoring of the microenvironment of the lipid bilayer at the molecular level. It may potentially be applicable to evaluate liposomal fusion, screen for penetration enhancers under development for enhancement in mucosal drug penetration, and monitor liposomal degradation within the living animal.