Definition and background10
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...
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