Hybrid systems combining liposomes and entangled hyaluronic acid chains: influence of liposome surface and drug encapsulation on the microstructure — en

In drug delivery, nanocarriers provide numerous advantages over conventional forms. Indeed, they can sustain drug release, target specific organs, increase therapeutic index, and protect poorly stable molecules. Among the nanomedicines on the market, liposomes are predominant due to their excellent safety profile and versatility. They are made of lipid bilayers surrounding one or several aqueous compartments, enabling the encapsulation of both lipophilic and hydrophilic molecules. Besides, their size (50 nm to 5 microns) and surface are easily tunable.

These liposomes are often incorporated within hydrogels to design locally injectable drug delivery systems that enable targeting local therapies for some tissues that are hardly accessible from the bloodstream. A strategy for designing such hydrogels is to mix the liposomes with hyaluronic acid (HA). This linear polyanionic polymer is naturally present in the human body, highly biocompatible, and biodegradable. Due to their shear thinning behavior, concentrated solutions of HA are easily injectable through fine needles.

From a therapeutic point of view, mixtures of HA with liposomes have shown their efficacy in otology and ophthalmology. However, little is known about their microstructure. It prompted us to perform an exhaustive study on HA/liposome mixtures. We assessed the influence of liposome surface properties (neutral, cationic, anionic, or anionic and covered with poly(ethylene glycol)), HA concentration, and encapsulation within liposomes [1] of a small corticoid (dexamethasone phosphate)[1]. We combined Small Angle Neutron Scattering with several complementary methods (cryo-electron microscopy, dynamic light scattering, size exclusion chromatography, and rheology).

The first important finding is that liposomes keep their integrity in HA, whatever the conditions probed. Second, entropic effects and the interactions of HA chains with the liposomes’ surface drive the dispersion of liposomes within the HA network. Indeed, when there are repulsions or no electrostatic interaction between both objects, a depletion mechanism leads to the substantial aggregation of the liposomes. Anionic and PEGylated liposomes form dense clusters with an amorphous organization, whether the corticoid is or not encapsulated (Figure 1.a). The corticoid encapsulation does not modify this organization. Conversely, cationic liposomes form less dense aggregates and are better dispersed due to their electrostatic complexation with HA chains (figure 1.b).