Novel methods for liposome preparation

Highlights

• New methods of liposome synthesis and modifications to conventional methods reviewed.

• Methods compared based on type of liposomes and amenability to scale-up and scale-down.

• Outlook on the influence of current and future technologies on liposome preparation methods.

Abstract

Liposomes are bilayer vesicles which have found use, among other applications, as drug delivery vehicles. Conventional techniques for liposome preparation and size reduction remain popular as these are simple to implement and do not require sophisticated equipment. However, issues related to scale-up for industrial production and scale-down for point-of-care applications have motivated improvements to conventional processes and have also led to the development of novel routes to liposome formation. In this article, these modified and new methods for liposome preparation have been reviewed and classified with the objective of updating the reader to recent developments in liposome production technology.

Introduction

Liposomes are spherical vesicles having an aqueous core enclosed by one or more phospholipid bilayers or lamellae. Liposomes are most frequently classified on the basis of their size (small, large and giant vesicles), number of bilayers (uni-, oligo- and multi-lamellar) (Vemuri and Rhodes, 1995, Vuillemard, 1991) and phospholipid charge (neutral, anionic or cationic) (Storm and Crommelin, 1998). Recently, liposomes have also been categorized with respect to their function such as conventional, stealth, ligand-targeted, long-release, and triggered-release (Sharma and Sharma, 1997, Storm and Crommelin, 1998). Multi-functional liposomes possessing a combination of these features have also been reported (Kale and Torchilin, 2010, Perche and Torchilin, 2013, Xiang et al., 2013). Even though liposomes were first reported close to half a century ago (Bangham et al., 1965), it was three decades after their discovery that the first liposomal drug Ambisome^® entered the market (Davidson et al., 1994, Hann and Prentice, 2001). Since then, the list of liposomal drugs has continued to increase (Allen and Cullis, 2013).

Over the years, different recipes for liposome preparation at the laboratory scale have been developed and optimized. Liposome preparation techniques may be divided into (a) bulk methods, where liposomes are obtained by transfer of phospholipids from an organic phase into an aqueous phase, and (b) film methods, in which lipid films are first deposited on a substrate and subsequently hydrated to give liposomes. The preparation methods have also been classified based on mean size, polydispersity and lamellarity of liposomes obtained, because control over these parameters remains a challenge with almost all preparation methods. This problem is exacerbated when moving from the laboratory to industrial scale (Mozafari, 2005, Riaz, 1996, Wagner and Vorauer-Uhl, 2011). For drug delivery applications the desirable size of liposomes ranges between 50 and 200 nm (Harashima et al., 1994, Woodle, 1995). Therefore, reduction of size and lamellarity of liposomes is typically carried out by subjecting them to homogenization, sonication, extrusion or freeze–thaw cycles (Brandl et al., 1990, Furukawa, 2010, Hope et al., 1985, Hope et al., 1986, Johnson et al., 1971).

Several exhaustive reviews have been published describing and comparing conventional methods for liposome preparation (Akbarzadeh et al., 2013, Dua et al., 2012, Mansoori, 2012, Mozafari, 2005, Riaz, 1996, Sharma and Sharma, 1997, Shashi et al., 2012, Storm and Crommelin, 1998, Wagner and Vorauer-Uhl, 2011). The present review covers these methods only briefly and further details may be found in cited literature. The review also does not cover the vast literature on synthetic modifications to phospholipids that are aimed at avoiding liposome detection and clearance by the immune system, targeting of liposomes to afflicted sites and controlled drug release. The focus of this review is on novel methods of liposome preparation that have become possible due to recent developments in technologies enabling fabrication and pattern formation on materials at the micro- and nano-scale. The review also discusses innovative strategies that have been used to overcome the limitations faced in conventional methods of liposome preparation. However, one should bear in mind that the new liposome preparation methods discussed in this review have only been demonstrated at the laboratory scale and remain to be developed for industrial use. Prior to describing the liposome preparation techniques we mention some commonly used liposome characterization methods and briefly discuss the forces and mechanisms governing liposome formation.