Liposomes were discovered in the early 1960’s and subsequently studied as cell membrane models. They have since gained recognition in the field of drug delivery. Liposomes are spherical vesicles which can be thought of as a hollow sphere they are composed of a bilayer membrane which entraps an aqueous core. The particle size of liposomes ranges from 20 nm to 10 μm in diameter. Liposomes vary in charge and in size depending on their manufacturing protocol and type of (phospho) lipid bilayer used (the small unilamellar vesicle (SUV) size range is 0.02 -0.05 μm, the large unilamellar vesicles (LUV) size range is greater than 0.06 μm and the multilamellar vesicle (MLV) size range is 0.1 – 0.5 µm). The physicochemical characteristics of the liposomes, like particle size, lamellarity, surface charge, sensitivity of pH changes and bilayer rigidity can be modified. Liposomes showed promising result in the drug delivery but their applicability is limited primarily to specific use because of short half-life in blood circulation. The circulation time of liposomes in the blood stream is dramatically increased by attaching polyethylene glycol (PEG) – units to the bilayer, known as long circulating (Stealth) liposomes.
Main advantages of liposomes are that they offers suitable means for delivering drugs combined with the potential of improving the therapeutic index while greatly reducing the side effects. Liposomes are potential carrier for controlled drug release of tumors therapeutic agents and antibiotic, for gene and antisense therapy through nucleic acid sequence delivery, immunization through antigen delivery and for antiparkinsons. Last one decade, pharmaceutical researcher’s use the tools of biophysics in evaluating liposomal dosage forms. Liposomes have covered predominantly medical, albeit some non-medical areas like bioreactors, catalysts, cosmetics and ecology. The structure is known as a phospholipid bilayer of lamellar, is shown in Figure 6.
Fig 6: The formation of liposomes, from phospholipid molecules to a unilamellar vesicle.
Composition of Liposomes:
There are number of the structural and nonstructural components of liposomes, major structural components of liposomes are:- a. Phospholipid: Phospholipid is the major component of the biological membrane; two types of phospholipids are used natural and synthetic phospholipids. The most common natural phospholipid is the phospatidylcholine (PC) is the amphipathic molecule and also known as lecithin. It is originated from animal (hen egg) and vegetable (soyabean). The type of phospholipids includes phosphoglycerides and sphingolipids, and together with their hydrolysis product. . PC is amphiphilic and is composed of a hydrophilic head group consisting of the quaternary ammonium moiety choline linked to the glycerol-backbone via a phosphor-ester and two lipophilic acyl chains. As the phosphate is negatively charged at physiological pH, PC is zwitter ionic and liposomes made of it have no net charge. PC is hardly ever used alone in liposomal lipid formulations. Blends of PC with other lipids are used primarily to improve both in-vitro and in-vivo stability of the liposomes. When drugs are incorporated into the liposome one usually wants to prevent leaking and loss of drug through the membrane. A schematic presentation of PC is shown in Figure 7.
Fig 7: A schematic representation of PC
Incorporation of cholesterol in liposome bilayer can bring about big changes in the preparation of these membranes. It does not mean by itself form bilayer membrane structure, but can be incorporated into phospholipids membrane in very high concentration up to 1:1 or 2:1 molar ratios of cholesterol to phospatidylcholine. Being an amphipathic molecule, cholesterol inserts into the membrane with its hydroxyl group of cholesterol oriented towards the aqueous surface and aliphatic chain aligned parallel to the acyl chains in the center of the bilayers and also it increase the separation between choline head groups and eliminates the normal electrostatic and hydrogen bonding interaction. A normal way to prevent leaking is adding cholesterol to the membrane, cholesterol will induce a tighter packing of the membrane and reduce the fluidity of the membrane, as shown in figure 8.
Fig 8: Phospholipid bilayer with cholesterol incorporated in the membrane
Materials can either be entrapped in the aqueous core or incorporated within the membrane. Lipophilic of amphiphilic drug are incorporated into the membrane and hydrophilic drugs are entrapped in the aqueous core. A multi lamellar liposome is shown in Figure 9; there are many phospholipid bilayers with water in between the layers. The pink dots are water-soluble drugs which are entrapped in the core or in the aqueous space between the bilayers. The green rods are lipid-soluble drugs which are incorporated in the lipid membrane.
Fig 9: Drug encapsulation in liposomes, the water-soluble drugs (shown in pink) are entrapped in the aqueous compartments and the lipid-soluble drugs (shown in green) are entrapped within the membrane. Due to recent developments in liposomes technology, more effective strategies are now available for controlling the stability and reactivity of liposomes after systemic administration. On the basis of ability of liposomes to interact with cells and or blood components, at least two types of liposomes currently can be designed including. (a)Non-interactive sterically stabilized (long-circulating) liposomes (LCL) and; (b) Highly interactive cationic liposomes.
Classification of liposomes9
Liposomes are often classified according to their size. Size and lamellarity of liposomes formed by spontaneous swelling depend on the type of lipid, composition of the medium and the mechanical stress exerted during swelling. Lipids with a net charge reduce both size and number of lamellae of the liposome. The vesicle size is critical parameter in determining circulation half-life of liposomes, and both size numbers of bilayers influence the extent of drug encapsulation being classified in Table 1. S.No.TypeSize RangeCharacteristics
1Multilamellar Vesicles 0.1-0.3µmMore than one bilayer
AOligolamellar Vesicles0.1-0.3µmIntermediate between LUV and MLV
BMultivesicular Liposomes0.1-0.3µmSeparate Compartment are Present in Single MLV CStable Plurilamellar Vesicles0.1-0.3µmHave Unique Physical and biological properties due to Osmotic Compression 2Large Unilamellar Vesicles0.1-10 µmSingle bilayer ,rapidly cleared by RES, 3Small Unilamellar Vesicles≤ 0.1 µmHomogenous in Size, Thermodynamically Unstable Table 1: Classification of Liposomes
The methods of preparation have been classified to the three basic modes of dispersions. •Physical dispersion involving hand shaking and non-hand shaking methods, •Solvent dispersion involving ethanol injection, ether injection, double emulsion vesicle method, reverse phase evaporation vesicle method, and stable plurilamellar vesicle method. •Detergent solublization
Liposomal formulations have been designed to address specific obstacles and functions as drug delivery systems. These formulations often vary in lipid composition and may include surface modifying groups. For convenience, liposomes can be classified into six main classes: conventional liposomes, targeted liposomes, viral fusogenic liposomes, pH-sensitive liposomes, cationic complexes and sterically stabilized liposomes. Liposome has a benefit that they can incorporate both kinds of drugs hydrophilic as well as lipophilic depicted in Figure 10. Fig 10: Nature of drug and their site of incorporation in liposomes Advantages of Liposomes10
1. Liposomes are biocompatible, completely biodegradable, non-toxic, flexible and nonimmunogenic for systemic and non-systemic administrations. 2. Liposomes supply both a lipophilic environment and aqueous “milieu interne” in one system and are therefore suitable for delivery of hydrophobic, amphipathic and hydrophilic drugs and agents. 3. Liposomes have the ability to protect their encapsulated drug from the external environment and to act as sustained release depots (propranolol, cyclosporin). 4. Liposomes can be formulated as a suspension, as an aerosol, or in a semisolid form such as gel, cream and lotion, as a dry vesicular powder (proliposome) for reconstitution or they can be administered through most routes of administration including ocular, pulmonary, nasal, oral, intramuscular, subcutaneous, topical and intravenous.
5. Liposomes could encapsulate not only small molecules but also macromolecules like superoxide dismutase, hemoglobin, erythropoietin, interleukin-2 and interferon-g. 6. Liposomes reduce toxicity and have increased stability of entrapped drug via encapsulation. (amphotericin B, taxol) 7. Liposomes showed increased efficacy and therapeutic index of drug (actinomycin-D). 8. Liposomes help to reduce exposure of sensitive tissues to toxic drugs. 9. They alter the pharmacokinetic and pharmacodynamic property of drugs (reduced elimination, increased circulation life time). 10. They exhibit flexibility to couple with site-specific ligands to achieve active targeting (anticancer and antimicrobial drugs).