Liposomes as Tools in Basic Research and Industry (1994)

Jean R. Philippot, Francis Schuber

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351360869
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Cell Biology
• Index: Biological Sciences

This book is devoted to a broader understanding of liposomes as a versatile tool used in many domains, including basic research and applied technology, focusing on less common applications and recent developments.
Over the past few years, new types of liposomes made of nonphospholipid molecules have opened new perspectives in applications. These lipid vesicles, already used in cosmetology, are being manufactured for industrial and agricultural uses. However, "Stealth" liposomes, pH-sensitive liposomes, and cationic liposomes have enlarged and improved the application field of liposomes in clinical research. The book covers these different uses of liposomes with particular attention to new formulations and new applications.

PART I
The Liposomes

Chapter 1

Influence of Liposome Characteristics on Their Properties and Fate

Roger R.C. New

CONTENTS

I. Introduction

II. Liposome Properties

A. Choice of Lipid

1. Phospholipid (PL) or Non-Ionic Surfactant (NIOS)?

2. Synthetic or Natural?

3. Saturated or Unsaturated?

4. Inclusion of Cholesterol?

5. Charged Lipids

6. Alternative Lipids?

B. Control of Liposome Size

III. Conclusion

IV. Summary of the Basic Principles of Liposomology

A. Preparation

B. Properties of Liposomes

C. Stability and Storage

D. Biological Behavior

1. Proteins

2. Cells

3. Whole Body

V. Glossary

A. Quantification of Lipid

1. Entrapment

2. Entrapped Volume

3. Capture Efficiency

4. Percentage Entrapped

B. Lipids

1. Membrane-Forming Lipids

2. Phospholipids

C. Membrane Behavior

1. Lamellar Phases

2. Transition Temperature

3. Membrane Dimensions

4. Discontinuities

5. Phase Separation

6. Asymmetry

7. Aggregation

8. Permeability

9. Fusion

D. Types of Liposomes

1. Size

2. Morphology

3. Method of Preparation

4. Composition

5. Function

I. Introduction

Three key elements serve to define liposomes as unique: (1) they are closed vesicles, (2) they enclose an internal aqueous space, and (3) this internal compartment is separated from the external medium by a bilayer membrane composed of discrete lipid molecules. Aside from this, liposomes can be any size, shape, and composition; it is the variety of different qualities that liposomes can possess, while still conforming to the basic definition, that makes them such a powerful tool in a wide range of disciplines.

From the above, it is clear that liposomes are essentially particulate systems, which can be clearly distinguished from other classes of microparticle often used for similar purposes. They are thus distinct from oil-in-water emulsions in that the latter do not contain an aqueous phase, and the layer of surfactant at the interface between the two phases is a monolayer rather than a bilayer. Even a double emulsion consisting of water-in-oil-in-water phases fails to meet the third criterion, although, as the quantity of oil phase is progressively reduced, the boundary between emulsion and liposomes becomes decidedly blurred. Other structures, which viewed from the outside might appear identical to liposomes but possess a significantly different internal make-up, arise as a result of depositing a coat of phospholipids onto the hydrophobic surface of solid, polymeric spheres, in which the bulk of the internal phase is aqueous. Although lipid-coated nylon microcapsules and SupraMolecular BioVectors (SMBVs) will not be included here as classical liposomes, much of what is said in this chapter may apply to them, and the observation of differences between these entities and liposomes can throw light on how liposome structure influences the characteristic properties of liposomes.

Traditionally, the membrane components of liposomes have been phospholipids, particularly phosphatidyl cholines, partly because they are the building blocks that nature itself uses to form membranes and partly because the common phospholipids are lamella-forming lipids under all conditions and can do so easily without the aid of additional components. Such single component systems lend themselves to study without needing consideration of complicating factors such as interactions between different chemical species. Recently, however, bilayer membrane vesicles have been constructed using single-chain amphiphiles (e.g., UFAsomes) or non-ionic surfactants in which the principles of formation and physical properties are so similar to conventional liposomes that there is no reason why they should not be taken into the fold and treated as subsets of the general case.

Other subsets of liposomes, where the original concept is modified slightly, include those where the “bilayer” membrane is composed of membrane-spanning lipids in which the membrane components are long lipidic chains with polar moieties at either end. In theory, each molecule has one polar group at the internal surface of the membrane and the other at the external face, although in practice a number of molecules will have bent back upon themselves to be associated with one or other of the faces exclusively. A second subtype stretches the concept of discrete molecules making up the membrane, when the membrane compounds are chemically linked to each other by polymerization, to form an extensive network of macromolecules inextricably interlinked. To form these structures, however, a conventional liposome is first constructed from individual monomer units making up a fluid membrane, and the chemical cross-linking may be thought of as a natural extension of the non-covalent interactions between membrane components (van der Waals, hydrogen bonding, electrostatic), which are essential for maintaining stability of the membrane in the first place.

Finally, one should pay homage to the ultimate liposome, the single cell, which fits the definition given above and, indeed, has been the inspiration and raison d’être for much of the work that has been conducted with liposomes, as models for cells in a very much simpler form.

II. Liposome Properties

Breaking things down to their simplest form, the two properties of liposomes that one wants to control are membrane stability and vesicle size, because these are the overriding factors that govern longevity in vitro and interactions in biological or other systems. Membrane stability, which includes permeability, fluidity, and fusogenicity, is determined by choice of lipid composition, while the size of liposomes is governed by the method of preparation.

A. Choice of Lipid

The decisions as to what lipids to use can be taken sequentially, and an attempt to do so is given below, with the justification and implications explained for each possible decision. The determination of the composition is based on the principle that the bulk of the liposome membrane can be composed of a single neutral phospholipid acting as the structural “backbone” of the bilayer, against the background of which other “minor” lipids can be overlaid, to confer specific properties that may modulate the behavior in any desired fashion. The first choice, therefore, is the nature of this basic lipid.

Figure 1 Variations in structure of lipid portions of synthetic phospholipids.

1. Phospholipid (PL) or Non-Ionic Surfactant (NIOS)?

As far as the robustness of the membrane is concerned, there appears to be little difference between liposomes made with either of these components. This is not too unexpected since the interactions between the lipid chains should be similar, and attachment of PEG and polysaccharides (typical headgroups used in NIOSs) to the surface of phospholipid vesicles is now becoming a common practice. It would be surprising, however, if the differential permeability characteristics displayed by PL membranes to ions such as Na⁺ and Cl⁻ were to be mirrored by NIOS membranes, since such properties must be related to the net dipole across PL membranes, which is the result of orientation of the ester carbonyls and the polar headgroups.

Choice between the two will therefore depend on availability, cost, the desirability or otherwise of using natural products (as are phospholipids) and, for pharmaceutical applications, regulatory considerations. While there is no consensus as to what NIOS is most suitable among those used, the phospholipid of choice is phosphatidyl choline (PC), which is the most readily available and which is the predominant lipid in natural cell membranes. In what follows, the assumption will be made that phosphatidyl choline of some sort is being used as the staple membrane lipid.

2. Synthetic or Natural?

Use of synthetic lipids enables one to work with pure well-characterized materials—usually single species, perhaps displaying special qualities that are unavailable with natural lipids. They are good for performing controlled studies, but the relationship between the results obtained and the behavior of natural membranes is not always a close one. Natural membranes, and the phospholipids derived from them, are in fact a mixture of different species, each molecule consisting of permutations of two fatty acids on the two distinct positions on the glycerol bridge, out of a total pool of 10 to 15 fatty acids varying in chain length, saturation, and in some cases position of double bonds along the chain. In natural membranes, a preference is usually displayed for “mixed” phospholipids—i.e., ...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Preface
• The Editors
• Contributors
• Contents
• PART I. THE LIPOSOMES
• PART II. LIPOSOMES AS CELL OR MEMBRANE MODELS
• PART III. NEW DEVELOPMENTS OF LIPOSOMES
• Index