Gelation of the internal core of liposomes as a strategy for stabilization and modified drug delivery II. Theoretical analysis and modelling of in-vitro release experiments

PEG-DMA was incorporated in unilamellar liposomes. PEG-DMA crosslinking by photo-induced radical reaction transforms the liquid aqueous core of the liposome into a hydrogel. The molecular weight of PEG-DMA significantly influences both structural and release properties of these hybrid nanosystems, by affecting both membrane permeability and diffusional properties of the inner core. Release studies of 5-(6) carboxyfluorescein from Conventional Liposomes (CL) and Gel-in-Liposome (GiL) systems were carried out in a vertical Franz Diffusion Cell. A detailed transport model is proposed, aimed at describing the entire drug diffusive pathway from the vesicles' inner core, through the double-layer membrane, into the buffer solution in the donor chamber of the Franz Cell and from there to the receptor chamber, where withdrawals are performed to evaluate the released drug concentration. The model permits to give a quantitative estimate of the diffusional resistances offered by the inner core (liquid or gelled) and by the double-layer membrane for CLs and different GiLs systems. The theoretical analysis of experimental release data strongly supports the basic assuption that, by varying the molecular weight of PEG-DMA, a different arrangement of the polymer within the liposomal structure and a different interaction with the membrane occur. PEG750-DMA decreases the transport resistance of the double layer membrane with respect to CLs, while PEG4000-DMA plays the opposite role. After gelation of the internal core, the diffusional resistance to drug transport inside GiLs becomes controlling, thus significantly slowing down drug release from these systems. Therefore, the combination of PEG-DMA with phospholipid vesicles appears an interesting strategy to develop sustained drug delivery systems.