[ACS Symposium Series] Polymeric Drugs and Drug Delivery Systems Volume 469 || Liposomes

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  • Chapter 3

    Liposomes

    Naoto Oku 1

    Faculty of Pharmaceutical Sciences, Setsunan University, Hirakata, Osaka, Japan

    Liposomes represent an important class of carrier vehicles other than polymers for drug delivery. This paper provides an introduction and general review of liposomes with emphasis on their classifications, their constituent materials, their preparation and characterizaton, and their stability and biodistribution in the body. Liposomes with specific characteristics are also described in this general introduction.

    Phospholipids derived from biomembranes when dispersed in water form lipid bilayer vesicles named liposomes which enclose an aqueous space inside the vesicle (Figure 1). Drugs and other substances can be entrapped either in the internal aqueous phase of the liposomes or in the lipid bilayers depending on their hydrophilicity. Liposomes are being used as carrier vehicles for drugs, such as antifungal and anticancer drugs and represent a new drug delivery system. When liposomes are used as drug carriers, it is possible to target the delivery site, enhance and sustain the clinical effects, reduce the toxicity of drugs, and protect drugs from metabolism and immune response (1-7).

    Liposomes are formed due to the amphiphilic character of lipids which assemble into bilayers by the force of hydrophobic interaction. Similar assemblies of lipids form microspheres when neutral lipids, such as triglycerides, are dispersed with phospholipids. Liposomes are conventionally classified into three groups by their morphology, i.e., multilamellar vesicle (MLV), small unilamellar vesicle (SUV), and large unilamellar vesicle (LUV). This classification of liposomes is useful when liposomes are used as models for biomembranes. However, when liposomes are used as capsules for drugs, size and homogeneity of the liposomes are more important than the number of lamellars in a liposome. Therefore, "sized" liposomes are preferred. These are prepared by extrusion through a polycarbonate

    Current address: School of Pharmaceutical Sciences, University of Shizuoka, Yada, Shizuoka 422, Japan

    0097-6156/91/0469-0024$06.00/0 1991 American Chemical Society

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    In Polymeric Drugs and Drug Delivery Systems; Dunn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

  • 3. OKU Liposomes 25

    filter with distinct pore sizes (8). Shown in Table I are several kinds of liposomes, which differ in size, lamellar number and preparation method.

    Multilamellar vesicles (MLVs) are easily prepared by simple hydration of phospholipids, and are rather stable. MLVs consist of a series of concentric bilayers separated by an aqueous compartment. If M L V s are administered by intravenous injection (iv), they tend to be taken up by reticuloendothelial systems (RES), such as the l iver and spleen. M L V s are suited for sustained release of drugs by subcutaneous or intramuscular administration. Small unilamellar vesicles (SUVs) can be prepared homogeneously with diameters of 20-100 nm. SUVs tend to be retained in blood for longer periods when administered iv. Although extremely large molecules cannot be entrapped in SUVs, materials less than 20,000 in molecular weight can be entrapped and at the same trapping efficiency (75). SUVs specifically composed of phospholipids with saturated fatty acyl chains (saturated phospholipids), have a tendency to fuse together (76). Large unilamellar vesicles (LUVs) have a larger internal volume and can be used to encapsulate huge materials such as bacteriophages (77). The diameters of LUVs are 100-1000 nm.

    Liposomes and Their Constituent Lipids

    Shapes of Lipids. Not all lipids derived from biomembranes prefer to form lipid bilayers. Membrane lipids can be classified into three groups by their shape, namely relative volume of polar head group and the size of the hydrophobic acyl chain (75). The first group consists of cylindrical shaped lipids, such as phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylglycerol (PG), and sphingomyelin (SM). These lipids tend to form stable bilayers in aqueous solution, and therefore, they are designated bilayer forming lipids. Phosphatidylcholines (PC) are most commonly used for making liposomes. PC liposomes are quite stable against the changes in pH or salt concentration.

    The second group cons i s t s o f cone - shaped l i p i d s , such as phosphatidylethanolamine (PE) and cardiolipin (CL) which have a large hydrophobic tail against the polar head groups. These lipids are called nonbilayer lipids, and form a hexagonal II structure as well as bilayer structure depending on the conditions (79). Nonbilayer lipids are thought to be important for membrane fusion (20). The third group of lipids consists of wedge-shaped lipids. Lyso-phospholipids are classified into this group. Liposomal lipids should be selected according to the purpose. For the purpose of sustained release of encapsulated drugs, bilayer forming lipids are used for making liposomes. On the other hand, nonbilayer lipids are preferable for making fusogenic liposomes.

    Charges of Lipids. Natural phospholipids have neutral or negative charges at neutral pH. Charged lipids incorporated into liposomal membrane prevent liposomal aggregation, and expand the aqueous phase between lamellars of M L V resulting in an increase of the trapped volume (27). Membrane charge affects the permeability and trapping efficiency of encapsulated drugs or materials. It also affects the biodistribution of liposomes. Stearylamines (positive), PS, PG, and

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    In Polymeric Drugs and Drug Delivery Systems; Dunn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

  • 26 POLYMERIC DRUGS AND DRUG DELIVERY SYSTEMS

    j a r

    Figure 1. Structure of liposomes and lipid microsphere a), multilamellar vesicle b). unilamellar vesicle c). lipid microsphere. Symbols inside the microsphere indicate di- and tri-acyl glycerol.

    Table I. Classification of Liposomes

    Abbreviation and Description of Liposomes References

    M L V s Multilamellar vesicles: easy to make, stable

    SUVs S m a l l un i l ame l l a r ves i c l e s : 20-100 nm, homogeneous

    LUVs Large unilamellar vesicles: 100-1000 nm, large trapped volume

    GUVs Giant unilamellar vesicles: >1 | im, huge trapped volume, unstable 9,10

    REVs Reverse-phase evaporation vesicles: L U V s prepared by reverse-phase evaporation method, high encapsulation efficiency 11,12

    FTVs Freeze-thaw vesicles: L U V s prepared by freeze-thaw method, high encapsulation efficiency

    VETs Vesicles by extrusion techniques: homogeneous sized vesicles 8

    SPLVs Stable plurilamellar vesicles: M L V s without osmotic unbalance 13

    MPVs Monophasic vesicles: S P L V s prepared with water-miscible solvent 14

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  • 3. OKU Liposomes 27

    phosphatydic acid (PA) (negative) are usual components of liposomal membranes in 10-20 mole % to endow charges.

    Phase Transition of Lipids. Lipids with saturated acyl chains form a solid bilayer (gel phase) at lower temperatures where the mobility of hydrocarbon chains is suppressed. This bilayer undergoes a phase transition with an increase in the temperature, and enters a fluid phase (liquid crystalline phase above the phase transition temperature (T c). The T c increases with an increase in fatty acyl chain length, and decreases with unsaturation of the acyl chain. If phospholipids have the same fatty acyl chain, PA, PS, and PE have a higher T c than PC or PG due to the hydrogen bonding between the polar head groups (22). A liposomal membrane is fluid and permeable at a liquid crystalline phase compared with that of a gel phase. Liposomes composed of saturated phospholipids lose barrier function during phase transition (23). This character, allows drugs encapsulated in liposomes composed of dipalmitoyl PC (DPPC) and distearoyl PC (DSPC) to be released at warmer target sites (24)

    Effect of Cholesterol. Cholesterol inclusion into the lipid bilayers composed of DPPC or DSPC, eliminates apparent T c and reduces permeability at and above the usual T c . On the other hand, cholesterol inclusion increases packing of fluid bilayer composed of lipids with unsaturated fatty acyl chains. Since cholesterol rich liposomes are stable in plasma, cholesterol is commonly used as a liposomal component.

    Preparation of Liposomes

    Typical M L V s are prepared by simple hydration of lipids in an aqueous solution with mechanical agitation using a vortex mixer. If the lipids are hydrated without agitation, giant oligo- or uni-lamellar vesicles are produced (9,25). Once liposomes are prepared by hydration and then mechanically broken, they reform closed vesicles again with changes in size and lamellar number. SUVs are produced by mechanical breakdown of M L V s using sonication or a French press. Homogeneous sized liposomes are produced by extrusion through a polycarbonate membrane with pores of defined sizes. For example, SUVs are produced by continuous extrusion of M L V s through a 100 nm pore (8,26). To make a large quantity of smaller sized liposomes, a microemulsification method has been developed (27). Rearrangement of lipid bilayers also occurs by a dehydration-rehydration process, such as freeze-thawing. When a SUV solution is frozen, SUVs fuse together due to ice growth, and rehydrated lipids form L U V s or giant unilamellar vesicles (GUVs) after thawing (2829). Procedures for producing liposomes by simple hydration with or without rearrangement of lipid bilayers are suitable for mass production of liposomes and for liposomal manufacturing.

    Other methods for preparing liposomes have been developed, such as hydration of lipids from an organic phase, and detergent removal methods. A lipid bilayer is formed in an aqueous solution of lipids dissolved in an organic solvent

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    In Polymeric Drugs and Drug Delivery Systems; Dunn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

  • 28 POLYMERIC DRUGS AND DRUG DELIVERY SYSTEMS

    when the solvent is removed or diluted. For example, L U V s are formed from a water-in-oil emulsion, followed by removal of solvent (77,72). This technique has a high "trapping" efficiency, and is called the reverse-phase evaporation method. Unilamellar vesicles are formed upon the removal of detergent from mixed micelles composed of detergent and phospholipids by dialysis, gel filtration, or centrifugation (30,31). These detergent removal methods are quite often used for reconstitution of membrane proteins in liposomes.

    Characterization of Liposomes

    Size and homogeneity of liposomes are important in their use as drug carriers, since these characters affect the pharmacokinetics and distribution of liposomes in vivo. Light scattering, electron microscopy, N M R , and gel filtration are used for evaluating liposomal size and homogeneity (32,33)

    Liposomes entrap hydrophobic materials in the lipid bilayer, and encapsulate hydrophilic materials in the aqueous inner phase. In the latter case, encapsulation depends on the volume of aqueous phase. This volume is called "trapped volume" and is designated in liters per mole of lipid. Trapped volume is usually measured by encapsulation of small aqueous markers. The encapsulation efficiency is a measure of how much material or drug is entrapped during the preparation of liposomes and is dependent on the preparation method, lipid concentration, and trapping material. The efficiency of trapping hydrophobic materials in the lipid bilayer is usually high. Liposomes formed by the freeze-thawing method also have a high trapping efficiency, e.g., nearly 90%, due to concentration of solute during freezing (34,35). In some experiments, an external material is entrapped in preformed liposomes. This is called active loading, and enables about 100% trapping of solutes (36 J7)

    Stability and Biodistribution of Liposomes

    The fate of injected liposomes is drastically altered by administration route, dose and size, lipid composition, surface modification, and encapsulated drugs. Liposomes encapsulating drugs are often administered iv, therefore, the stability of liposomes in plasma is important. When liposomes composed of PC with unsaturated fatty acyl chains are incubated in the presence of serum, an efflux of internal solute from the liposomes is observed. This increase in permeability is caused by the transfer of phospholipids to high density lipoprotein (HDL) in serum (55). To reduce the efflux of liposomal contents, cholesterol is added as a liposomal component

    The half-life of liposomes administered in the blood stream is affected by the composition, size, charge, and fluidity. Liposomes with a small size or with a rigid lipid bilayer have a longer half-life (389). Large liposomes administered iv tend to accumulate at a lymph node near the injected site. This tendency can be useful for preventing metastases. Liposomes which pass through the lymph node have a tendency to accumulate in the RES, such as the liver and spleen (40,41). The disposition of liposomes is altered by the dose of liposomes as well as size or lipid composition of liposomes. Cholesterol rich liposomes are cleared slower due to

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    In Polymeric Drugs and Drug Delivery Systems; Dunn, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

  • 3. OKU Liposomes 29

    their low uptake by RES (42). Since the RES uptake of liposomes is a saturatable process, the clearance rate of liposomal drugs decreases with increasing liposome doses or pre-dosing with empty liposomes (43)

    Liposomes tend to remain at the injection site when they are administered intramuscularly or subcutaneously. Therefore, these administration routes are useful for slow and sustained release of drugs at the injection site.

    Liposomes with a Specific Character

    Many liposomes with a specific function, such as targeting, have been developed. Among them, four kinds of liposomes are described.

    Temperature-Sensitive...

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