The effect of neutral helper lipids on the structure of cationic lipid monolayers

A P Dabkowska,D J Barlow,R A Campbell,A V Hughes,M J Lawrence,P J Quinn

doi:10.1098/rsif.2011.0356

A P Dabkowska, D J Barlow + Show 4 more

Open Access

https://doi.org/10.1098/rsif.2011.0356

Copy DOI

Abstract

Successful drug delivery via lipid-based systems has often been aided by the incorporation of 'helper lipids'. While these neutral lipids enhance the effectiveness of cationic lipid-based delivery formulations, many questions remain about the nature of their beneficial effects. The structure of monolayers of the cationic lipid dimethyldioctadecylammonium bromide (DODAB) alone, and mixed with a neutral helper lipid, either diolelyphosphatidylethanolamine or cholesterol at a 1 : 1 molar ratio was investigated at the air-water interface using a combination of surface pressure-area isotherms, Brewster angle microscopy (BAM) and specular neutron reflectivity in combination with contrast variation. BAM studies showed that while pure DODAB and DODAB with cholesterol monolayers showed fairly homogeneous surfaces, except in the regions of phase transition, monolayers of DODAB with diolelyphosphatidylethanolamine were, in contrast, inhomogeneous exhibiting irregular bean-shaped domains throughout. Neutron reflectivity data showed that while the thickness of the DODAB monolayer increased from 17 to 24 Å as it was compressed from a surface pressure of 5-40 mN m(-1), the thickness of the helper lipid-containing monolayers, over the same range of surface pressures, was relatively invariant at between 25 and 27 Å. In addition, the monolayers containing diolelyphosphatidylethanolamine were found to be more heavily hydrated than the monolayers of cationic lipid, alone or in combination with cholesterol, with hydration levels of 18 molecules of water per molecule of lipid being recorded for the diolelyphosphatidylethanolamine-containing monolayers at a surface pressure of 30 mN m(-1) compared with only six and eight molecules of water per molecule of lipid for the pure DODAB monolayer and the cholesterol-containing DODAB monolayer, respectively.

Highlights

Cationic lipid-based vehicles have long been used for the delivery of genetic material into cells [1]
The effect of the helper lipids, cholesterol and dioleoyl phosphoethanolamine (DOPE), on dimethyldioctadecylammonium bromide (DODAB) monolayers was studied by determining their p– A isotherm when spread on an aqueous subphase containing 10 mM NaCl
The presence of either of the two neutral helper lipids studied induced significant changes in the structure of monolayers formed by the cationic lipid DODAB, which may have implications for the ability of the lipoplexes formed by these lipids to successfully deliver DNA

Summary

Introduction

Cationic lipid-based vehicles (known as lipoplexes) have long been used for the delivery of genetic material into cells [1]. It is widely recognized that the efficiency of nucleic acid delivery achieved using lipoplexes is limited, with the further problem that the cationic lipids employed are toxic to cells [2]. One of the lipids used in the preparation of such lipoplexes (and the one chosen for this study) is the commercially available lipid, dimethyldioctadecylammonium bromide (DODAB). This lipid has been shown to be successful in delivering nucleic materials in vitro when used as sole lipid [6], its effectiveness is increased in the presence of neutral helper lipids such as DOPE and the cholesterol derivative, PtdChol [7]. Previous studies have investigated the structure of a DODAB monolayer at the air – water interface [8 – 11], sometimes in the presence of DNA [12,13] but (to the authors’ knowledge) there have been no studies concerned with the effect of a helper lipid on the structure of the DODAB lipid monolayer

Objectives

Results

Conclusion