Interactions of Complementary PEGylated Liposomes and Characterization of the Resulting Aggregates

Abstract

The interaction of complementary liposomes bearing both recognizable and protective ligands at their external surface has been investigated. Aggregation of hydrogenated phosphatidyl choline/cholesterol (2:1 molar ratio) based liposomes was mediated by the molecular recognition of the complementary phosphate and guanidinium groups incorporated in separate unilamellar liposomes. The phosphate group was incorporated in the bilayer employing dihexadecyl phosphate, while the guanidinium moiety was introduced in the membrane through the incorporation of various guanidinium lipids. For the latter, anchoring ability and primarily introduction of a spacer group between their lipophilic part and the guanidinium group was found to affect the ability for molecular recognition. Also, poly(ethylene glycol) (PEG) introduced in both types of liposomes at various concentrations and up to 15% with respect to cholesterol modifies the interaction effectiveness and morphology of the obtained aggregates. Interaction of these complementary liposomes leads to large precipitating aggregates or fused liposomes, as shown by phase contrast microscopy and dynamic light scattering. Specifically, fusion of liposomes takes place under a nonleaking process involving lipid mixing, as demonstrated by calcein entrapment and resonance energy transfer experiments. Calorimetric parameters also correlate with the processes of aggregation and fusion. The interactions of non-PEGylated liposomes involve exothermic processes of higher enthalpic content than those of the PEGylated counterparts.

Interaction between complementary liposomes: a process leading to multicompartment systems formation

“…The size increases during the interaction of PC:CHOL:DOPG with PC:CHOL:DHP liposomes in water is shown in Figure 6. Fusion of complementary liposomes takes place under a non-leaking process involving lipid mixing as it was established by calcein entrapment and Resonance Energy Transfer experiments (Pantos et al, 2004). The thermodynamic Example IV.…”

“…In recent studies (Pantos et al, 2002;Pantos et al, 2004) in which the inhibitory role of the protective polyethylene glycol (PEG) coating on molecular recognition was investigated, liposomes based on hydrogenated PC and cholesterol were prepared, incorporating recognizable moieties at their surface. One type of liposomes incorporated DHP, whereas their complementary liposomes contained either octadecylguanidine hydrochloride (ODG),7,propyl] guanidine hydrochloride (ODPG),8,propyl] guanidine hydrochloride (DOPG), 9.…”

“…In an attempt to model the interaction of drugloaded liposomes with cells, the interaction of loaded unilamellar with multilamellar liposomes was investigated. In this experiment the same lipids, as in the previous experiment (Pantos et al, 2004) were used for the preparation of liposomes. However, one liposome of the interacting complementary liposomal pair was in this case multilamellar.…”

Structural Features of Interacting Complementary Liposomes Promoting Formation of Multicompartment Structures

“…In addition, recognizable guanidinylated liposomes with increased PEG content (hereafter referred to as enhanced PEGylated recognizable liposomes) were formed by incorporation of PEG-CHOL in an amount of 5 mol % relative to cholesterol. At this molar content it has been established [7,40] that effective interaction occurs between the complementary liposomes and leads to fusion. Therefore, the presence of long PEG chains at the interface of unilamellar liposomes is expected to further affect fusion and formation of multicompartment systems following their interaction with the complementary multilamellar liposomes.…”

“…By careful scrutiny, primarily of our own work, we discovered that these large aggregates, obtained as the outcome of molecular recognition, exhibit multicompartment structures. [1,4,[6][7][8]12] Our consideration of the multicompartment nature of these bilayer aggregates led us to propose a working hypothesis, [8] according to which molecular recognition of selected liposomal pairs induces the formation of multicompartment systems. On the basis of this hypothesis, we further investigate in the present study the structural features of new complementary liposomal pairs in order to establish the factors that favor formation of multicompartment systems and their optimal interaction ratios.…”