In vivo ESR studies on subcutaneously injected multilamellar liposomes in living mice

Purpose. An innovative, noninvasive, low-frequency electron spin resonance (ESR) spectroscopy method was applied and adapted to investigate the integrity of multilamellar liposomes from hydrogenated phospholipids after subcutaneous injection in living mice. Moreover, the fate of the injected liposomal preparations was examined, as well as the possibility to achieve a depot effect. Methods. Highly concentrated solutions of the spin probe 2,2,6,6-tetramethyl-4-trimethylammoniumpiperidine-1-oxyl-iodide (CAT-1; 138 mM) were encapsulated in liposomes. They were characterized by laser diffraction, and the liberation of spin probe was investigated by ESR spectroscopy. Results. Line shape changes allowed the differentiation between encapsulated and released CAT-1 after subcutaneous injection of liposomes. Multilamellar liposomes form a local depot at the site of injection. A sustained release of the spin probe from the depot was monitored by means of ESR. Whereas 40% of the spin probe was released within the first 96 h after administration, 60% remained in intact liposomes under the skin. No depot formation could be observed after injection of CAT-1 solutions, but a fast signal decrease due to systemic distribution and bioreduction of the nitroxide spin probe. Conclusions. Noninvasive analysis of liposomal integrity in living animals was successfully accomplished using a new L-Band ESR spectroscopy method. The liberation of CAT-1 from liposomes in vitro and in vivo was monitored by changes in the lineshape of ESR spectra and Heisenberg spin exchange. The significance of liposomal integrity for the formation of a localized drug depot effect was proved.