Effect of Formulation Method, Lipid Composition, and PEGylation on Vesicle Lamellarity: A Small-Angle Neutron Scattering Study.

Valeria Nele,Margaret N Holme,Ulrike Kauscher,Molly M Stevens,Michael R Thomas,James J Doutch

doi:10.1021/acs.langmuir.8b04256

Abstract

Liposomes are well-established systems for drug delivery and biosensing applications. The design of a liposomal carrier requires careful choice of lipid composition and formulation method. These determine many vesicle properties including lamellarity, which can have a strong effect on both encapsulation efficiency and the efflux rate of encapsulated active compounds. Despite this, a comprehensive study on how the lipid composition and formulation method affect vesicle lamellarity is still lacking. Here, we combine small-angle neutron scattering and cryogenic transmission electron microscopy to study the effect of three different well-established formulation methods followed by extrusion through 100 nm polycarbonate membranes on the resulting vesicle membrane structure. Specifically, we examine vesicles formulated from the commonly used phospholipids 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) via film hydration followed by (i) agitation on a shaker or (ii) freeze–thawing, or (iii) the reverse-phase evaporation vesicle method. After extrusion, up to half of the total lipid content is still assembled into multilamellar structures. However, we achieved unilamellar vesicle populations when as little as 0.1 mol % PEG-modified lipid was included in the vesicle formulation. Interestingly, DPPC with 5 mol % PEGylated lipid produces a combination of cylindrical micelles and vesicles. In conclusion, our results provide important insights into the effect of the formulation method and lipid composition on producing liposomes with a defined membrane structure.

Highlights

Liposomes are soft self-assembled structures that have been extensively used for drug delivery and biosensing applications.[1−3] They are vesicles comprising a phospholipid bilayer surrounding an inner aqueous cavity
To evaluate the effect of the formulation method on vesicle lamellarity, we studied three common liposome formulation methods, namely: (a) film hydration followed by agitation on a shaker (AS), (b) film hydration followed by 5 freeze-thaw cycles (FT), and (c) reverse-phase evaporation (REV)
Different formulation methods cannot be expected to yield the same vesicle populations, and their outcomes depend on the chosen lipid composition

Summary

■ INTRODUCTION

Liposomes are soft self-assembled structures that have been extensively used for drug delivery and biosensing applications.[1−3] They are vesicles comprising a phospholipid bilayer surrounding an inner aqueous cavity. A unilamellar vesicle model was used to fit the measured neutron scattering curves, and a bilayer thickness of 48 Å was obtained In this case, extrusion through a 100 nm membrane has the sole effect of reducing vesicle size, and the scattering data for vesicles prepared via the REV method followed by extrusion could be fitted using the same model (see the Supporting Information). Scattering data for the unextruded POPC vesicles prepared via the AS and FT methods were fitted using a Broad Peak model (Table S3) In this case, the vesicle radii are too large to capture their overall dimensions in our measured Q range. We prepared vesicles including a small mol % of DOPE-PEG2000 of DPPC, POPC, or DOPC via the FT method and extruded 35 times through a membrane with 100 nm pore size.

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES