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Several efforts have been focused on targeted drug delivery systems for delivering a drug to a particular region of the body for better control of systemic as well as local action. Liposomes have proven their efficiency as a choice of carrier for targeting the drugs to the site of action. The main reason for continuous research on liposomes drug delivery is they largely attributed to the fact that they can mimic biological cells. This also means that liposomes are highly biocompatible, making them an ideal candidate for a drug delivery system. The uses found for liposomes have been wide-spread and even include drug delivery systems for cosmetics. Several reports have shown the applicability of liposomal drug delivery systems for their safe and effective administration of different classes of drugs like anti tubercular, anti cancer, antifungal, antiviral, antimicrobial, antisense, lung therapeutics, skin care, vaccines and gene therapy. Liposomes are proven to be effective in active or passive targeting. Modification of the bilayer further found to increase the circulation time, improve elasticity, Trigger sensitive release such as pH, ultrasound, heat or light with appropriate lipid compositions. The present chapter focuses on the fundamental aspects of liposomes, their structural components, preparation, characterization and applications.
GITAM Institute of Pharmacy, Gandhi Institute of Technology and Management (GITAM), Deemed to be University, Rushikonda, Visakhapatnam, Andhra Pradesh, India
V.S. Vinai Kumar Tenneti
GITAM Institute of Pharmacy, Gandhi Institute of Technology and Management (GITAM), Deemed to be University, Rushikonda, Visakhapatnam, Andhra Pradesh, India
*Address all correspondence to: slankala@gitam.edu;, isrinivas2001@gmail.com
1. Introduction
Liposomes are microscopic vesicles containing aqueous volume enclosed by lipid bilayer membrane [1]. A.D. Bangham and R.W. Thorne first described about liposomes in 1964 when observed under electron microscope while analyzing phospholipids dispersion in aqueous environment [2]. They observed spontaneous arrangement of phospholipids into “bag-like” circular structures. Gerald Weissman, one of the colleagues of Bangham suggested the structures as liposomes [3]. This discovery helped as a multipurpose tool in several fields like biology, biochemistry and medicine. Liposomes gained popularity in vesicular research due their attributes of biocompatibility and similar structural features of biological cells (See Figure 1). The amount of drug loaded into the liposomes and the size of the liposomes play pivotal roles in the pharmacokinetic and pharmacodynamic parameters of the drug. The size scale of liposomes varies with typical a mean size of 100 nm. Due to their size and hydrophobic and hydrophilic character liposomes are promising systems for drug delivery.
Figure 1.
Structure of liposomes.
Several reports showed the applicability of liposomes for the safe and effective administration of therapeutic molecules of different classes like antitubercular, anticancer, antifungal, antiviral, antimicrobial, antisense, lung therapeutics, skin care, vaccines, genes etc. [4]. Liposomes have proven their commercial importance from the first product ‘Doxil’, a PEGylated doxorubicin liposomal formulation [5, 6] to the latest ‘Marqibo’, vincristine sulfate liposomal formulation [7, 8]. Liposome properties differ considerably with lipid composition, surface charge, size, and the method of preparation. The nature of phospholipid bilayer determines the ‘rigidity’ or ‘fluidity’ and the charge of the vesicles. Further modifications of bilayer help in modulation of circulation time, permeability, stimuli response drug release from the liposomes.
Liposome vesicles are composed of phospholipids as an important structural component of the bilayered membrane and cholesterol is the other component mostly stabilizes the membrane. The properties of liposomes depend on the nature of phospholipids that are being used [9].
2.1 Phospholipids
Phospholipids are amphipathic [10, 11] molecules present in membrane. They contain hydrophilic head and hydrophobic tail. The hydrophilic head has phosphorus molecule as phosphoric acid group, and two hydrophobic tails have long hydrocarbon chain groups (See Figure 2). Phosphoglycerides, phosphoinositides and phosphosphingosides are three classes of phospholipids [12].
Figure 2.
Structure of phospholipid molecule.
2.1.1 Phosphoglycerides
Phosphoglycerides are the mostly used phospholipids which contain three OH groups in glycerol moiety and among them two OH groups are linked to two fatty acids and phosphoric acid linked with one OH group. Phosphoglycerides are differed with their attached ‘polar head alcohol group’ esterified with phosphoric acid. All phosphoglycerides will have two nonpolar “tails” of fatty acid (C16 or C18) and among them one is saturated and other is unsaturated which always attaches to middle or β-hydroxyl group.
Lecithins (Phosphatidyl Cholines): Lecithin is synonym for phosphotidylcholine which is a phospholipid containing phosphate obtained either from yolk of egg or from soya beans. Lecithin contains unsaturated non polar fatty acids, glycerol and phosphoric acid attached to nitrogen base choline (See Figure 3).
Cephalins: Cephalins have similar basic structure to that of lecithins. The choline present in lecithin is replaced with ethanolamine or serine and examples are phosphatidyl ethanolamine (See Figure 4) and phosphatidyl serine (See Figure 5). Cephalin exists in α and β forms based on position of two attached fatty acids. The primary amino group present in ethanolamine is weak base compared to quaternary ammonium group of choline. Hence, cephalins are more acidic and less soluble in alcohol than lecithins.
Plasmalogens (Phosphoglyceracetals): Plasmalogens contains only 10% of phospholipids and are structurally same like other two phosphoglycerides with change of one fatty acid replaced with unsaturated ether. The nitrogen base attached to phosphoric acid of plasmalogens can be choline, ethanolamine or serine and hence, names are phosphatidal choline (See Figure 6), phosphatidal ethanolamine (See Figure 7) and phosphatidal serine (See Figure 8).
Figure 3.
Structure of lecithin.
Figure 4.
Structure of Phosphatidyl Ethanolamaine.
Figure 5.
Structure of Phosphatidyl serine.
Figure 6.
Structure of Phosphatidal choline.
Figure 7.
Structure of Phosphatidal ethanolamine.
Figure 8.
Structure of Phosphatidal serine.
2.1.2 Phosphoinositides (phosphatidyl inositols)
Phosphoinositides are phospholipids which have cyclic hexahydroxy alcohol called inositol attached to phosphoric acid. The phosphoinositides on hydrolysis gives glycerol, fatty acids, inositol and phosphoric acid with 1 or 2 or 3 moles. Because of this monophosphoinositide (See Figure 9), diphosphoinositide and triphosphoinositide (See Figure 10) are found. Phosphoinositides are glycolipids which contains carbohydrate residue.
Figure 9.
Structure of Monophosphoinositide.
Figure 10.
Structure of Triphosphoinositide.
2.1.3 Phosphosphingosides (=sphingomyelins)
Sphingomyelins are structurally different from that of other phospholipids by lacking glycerol moiety and presence of nitrogeneous sphingosine or dihydrosphingosine along with choline. These are electrically charged molecules with polar head phosphocholine.
2.2 Cholesterol
Cholesterol is lipid containing steroidal ring with attached hydroxyl group (See Figure 11). The OH group present in cholesterol is united with phosphate head group of the phospholipids on biological cell membrane to keep them firm and fluid [13]. Cholesterol has a molecular formula, C27H45OH. It is a white crystalline solid and is optically active.
Liposomes are classified [14] mainly by structure, method of preparation, composition with application, conventional liposomes and specialty liposomes (See Figure 12).
Liposomes are prepared by using different methods (See Figure 13) in which the drug is entrapped by either passive or active loading [15, 16].
Figure 13.
Methods used for the preparation of liposomes.
6.1 Passive loading methods
This loading technique is to load or encapsulate drug molecules before forming or during preparing liposomes [9]. During liposomes preparation when the lipid film dissolved in drug containing aqueous buffer then that hydrophilic or water soluble drugs is loaded at the centre of liposome vesicle. When lipophilic drugs added to lipid phase of liposome components then that lipophilic drug will load in between lipid bilayers. The unentrapped drug is removed using gel-filtration chromatography or dialysis for liposomal dispersion [17]. The drug loading is low for hydrophillic compounds and high for lipophilic compounds in passive loading method. Liposomal vesicles with large size will have superior drug loading than small sized vesicles [18]. Lipid composition always influence for better drug loading by passive loading method [19].
Passive loading includes four types of methods namely,
Mechanical Dispersion.
Based on replacing organic solvent (Solvent Dispersion Method).
Based on size change or combination vesicle.
Detergent removal methods
6.2 Active loading methods
Certain compounds which have both aqueous and lipid solubility and having ionisable groups can be loaded after formation of vesicles [20]. This type of method is called remote or active loading of drug molecules. In this remote or active loading several methods exist in preparing of liposomes. Doxil™ is one of the liposomal products prepared by this method [21, 22].
6.3 Mechanical dispersion methods
6.3.1 Hand shaking or non hand shaking of lipid based film by hydration
This method was first described by [1] and which is simple for liposomes formation with one limitation of having low drug loading. Phospholipids and cholesterol are to be dispersed in organic solvent and then evaporated by using rotary evaporator at low pressure and vacuum. When solvent is evaporated then a dry film will be formed on the wall of rota flask and that should be hydrated by using aqueous phase buffer. The lipids in film spontaneously gets swell when hydrated to form heterogeneous multilamellar liposomes (MLVs).
6.3.2 Micro-emulsification
Micro fluidizer helps in preparing small MLVs from liposomal dispersion [23]. Micro fluidizer pumps liposomal fluid at pressure of 10,000 psi through 5 μm orifice and by micro channels that directs two pathways of dispersion to colloid with high velocity. The large MLVs liposomal dispersion or organic medium containing lipids can also be passed through fluidizer. The dispersion collected is replaced through the micro fluidizer until vesicles with spherical dimensions obtained.
6.3.3 Sonication method
When the liposomal vesicles sizes are above 1 μm then sonication is done to reduce size to form SUVs or extrusion done with polycarbonate filters for producing smaller and uniform sized vesicles. Size reduction by ultrasonication for aqueous dispersion can be done mainly by bath or probe sonicators [24].
6.3.4 French pressure cell
The French pressure press breaks cells with appropriate conditions compared with ultrasound techniques [25]. French pressure press is advantage because sonication procedure degrades lipids, proteins and sensitive compounds. This cell can be used for dispersions with low volume of less than 40 ml and not applicable for high volume production batches. Hence, a scale-up-based strategy was established by using micro fluidization technique.
6.3.5 Membrane extrusion
Another method for liposomes downsizing is extrusion. The vesicles with force are passed through membranes with a lower pressure than french press. Extrusion studies using polycarbonate filters were done and performed on extrusion behavior and membrane properties [26, 27]. Lipex Biomembranes Inc., now called Northern Lipids Inc., invented extrusion vessel from milliliter to several liters. This Lipex extruder allows higher temperatures with jacketed mode. An alternative to this lipex extruder is Maximator device, which is continuous pumping system which was introduced by Schneider et al., 1994. The Maximator has glass vessel which is thermostable and connected directly to pneumatic piston pump. This method consists of preparing liposomes followed by freeze–thaw and finally extrusion. This long process and disadvantage because of high product loss.
6.4 Methods on fusion of preformed vesicle
6.4.1 Dried and reconstituted vesicles (DRVs)
This method follows freeze drying for empty SUVs to form powder (See Figure 14). Then that freeze dried powder is rehydrated with aqueous phase media containing materials that are to be entrapped. This dispersion contains solid lipids which are in subdivided form. Freeze drying organizes membrane structure when rehydrated with water to fuse and reseal vesicles. For preparing uni or olio lamellar vesicles of 1.0 μm or less in diameterthis method is used [28].
Figure 14.
Preparation of dried reconstituted liposomal vesicles.
6.4.2 Extrusion method by freeze thaw
This is extension to above DRV method and lipid film formed by film hydration is mixed with solute containing entrapped materials to form vesicles. The obtained dispersion extruded for three times after two freeze thaws, vortexed and again freeze thawed 6 times followed by 8 extrusions. Then liposomes get fused and forms large unilamellar vesicles, and this method mostly used for proteins encapsulation [29].
6.5 Solvent dispersion methods
In these methods the lipids and non-aqueous soluble drug are added in organic phase and then that lipid phase injected into aqueous phase.
6.5.1 Solvent injection method
In this solvent injection method (See Figure 15) lipids are added into organic phase (ethanol or ether or chloroform) and that lipid phase is injected into aqueous phase to obtain liposomes [30]. This method again sub divided two methods depending on solvent used.
Figure 15.
Solvent injection method.
6.5.1.1 Ethanol injection method
A lipid solution of ethanol is rapidly injected to a large quantity of aqueous buffer. This ethanol injection method gives small liposomes without any extrusion or sonication [31, 32]. This method has some disadvantages that the liposomes formed are very dilute, difficulty of ethanol removal from azeotropic mixture and possible inactivation of various biologically active macromolecules in the presence ethanol.
6.5.1.2 Ether injection method
This ether injection method is different to ethanol injection. Ether is aqueously insoluble and requires hot condition to remove solvent from liposomal dispersion. This method involves single jet injecting lipid phase containing ether into heated aqueous phase. A solution of lipids dissolved in diethyl ether or ether-methanol mixture is gradually injected to an aqueous solution of the material to be encapsulated at 55–65°C or under reduced pressure. The ether vaporizes and the dispersed lipid forms primarily unilamellar liposomes [33]. Ether injection method has advantage over ethanol injection method because the used ether is removed to obtain a concentrated liposomal dispersion and high entrapment efficiency.
6.5.2 Double emulsion vesicles
The double emulsions are prepared by controlling flow rates at three different phases (i.e. 2 aqueous phases and 1 oil/lipid phase) to form single drop of aqueous solution within a single drop of oil in a continuous aqueous phase (See Figure 16). A method by using glass capillary micro fluidic device was fabricated from double emulsion containing phospholipid vesicles [34].
Figure 16.
Double emulsion liposomal vesicles.
6.5.3 Reverse phase evaporation
The solvent containing lipid is taken in rota flask and evaporated by rotary evaporator kept under low pressure. The formed lipid film was nitrogen purged and then dissolved again in diethyl ether or isopropyl ether containing organic phase to form vesicles. This forms an emulsion and that formed emulsion is again evaporated at low pressure and forms semisolid gel. These obtained liposomal vesicles are called reverse phase evaporation vesicles (REV) [35, 36].
6.5.4 Stable plurilamellar vesicles
Stable plurilamellar vesicles (SPLVs) are prepared using method described by [37]. The phospholipid suspension is to be taken in round bottom flask and evaporated using rotary evaporator to form dried film. To the dried phospholipid suspension an aqueous phase (HEPES [N-2-hydroxyethylpiperazine-N9–2-ethanesulfonic acid] buffer) was added and mixed. The mixture is to be shaken in mechanical shaker and placed in bath sonicator with a nitrogen gas passed through it to facilitate evaporation during the sonication. The liposomes are to be suspended again in the HEPES buffer. The elimination of the nonencapsulated material was done by column filtration and three washes needed in HEPES buffer.
6.6 Detergent removal methods
6.6.1 Detergent dialysis
In liposomal preparations some bile salts and alkyl glycosides are used as detergents for solubilization of lipids in micellar systems. The shape, size of liposomal vesicles always depends on used detergent chemical nature, concentration and lipids [38, 39, 40]. The common procedures for removing detergent from micelles are by dilution method [41, 42], gel chromatography [43] and dialysis by hollow fibers [44] or membrane filters [45, 46]. By combining ethanol injection and cross flow injection a new technique was developed to form proteo liposomes [47]. Liposomes of size range 40–180 nm is seen when detergent solubilises lipids to yield micelles [48]. Other methods like calcium induced fusion [49], nanoprecipitation [50] and emulsion techniques [51, 52] are used for liposomes preparation. These classical methods need more solvents and very harmful to human health, also require complete removal of organic solvent.
The liposomes production extended by techniques such as heating method, super critical reverse phase evaporation, spray drying, freeze drying and modified ethanol injection technique.
7.1 Heating method
A new heating method was available for production of liposomes [53]. All materials are to be mixed in aqueous phase and then to it glycerol (3% v/v) is to be added which will increase the stability for lipid vesicles. The glycerol containing lipid mixture will be heated up to 120° C to form vesicles. The studies on TLC of lipids showed that there is no degradation at mentioned temperature [54].
7.2 Spray-drying
Spray-drying of lipid mixture and drug is one of the methods for large scale industry production. The lecithin and mannitol are to be dissolved in chloroform, sonicated in bath sonicator and spray dried using mini spray dryer to form dried liposomal vesicles. Spray-drying conditions of temperatures 120° C for inlet and 80°C for outlet and 1000 ml/hr. of air flow rate are to be maintained. The obtained spray dried product is to be rehydrated using aqueous phase [55]. The liposomes size depends on used aqueous phase volume for rehydration [55].
7.3 Freeze drying
Freeze drying is a new method for preparation of submicron sized liposomes which are sterile and pyrogen-free [56]. Freeze drying method depends on dispersion having lipids with water-soluble carriers like sucrose and mannitol. Sucrose and mannitol are dissolved in cosolvent system containing tert-butyl alcohol and water. Lyophilizer can be used for freeze drying. The conditions for freeze drying are first freezing to be maintained at −40° C for eight hours followed by 1° drying maintained at −40°C for 48 hours and then secondary drying maintained at 25°C for 10 hours. Mainly 20 pascal pressure is to be maintained in freeze drying chamber at drying process. After reconstitution with aqueous phase the prepared freeze dried product forms liposomal dispersion. The lipid/carrier ratio plays vital role in size and polydispersity of the liposomal dispersion [56].
7.4 Super critical reverse phase evaporation (SCRPE)
This is single step process for preparation of liposomes under supercritical carbon dioxide condition [57]. Liposomal dispersion is formed by emulsion formation with water mixed in ethanol, LR-dipalmitoylphosphatidylcholine, and supercritical carbon dioxide at stirring condition with required pressure. Transmission electron microscopy (TEM) results showed large unilamellar vesicles formed at size range 0.1 to 0.2 μm [57]. The entrapment efficiency results also showed that five times more drug was entrapped by this method compared with Bangham method [57]. Results showed SCRPE is one best technique with single step for large single lamellar vesicles with good entrapment efficiency [58, 59].
7.5 Modified ethanol injection method
Novel approaches are available for liposomes production with principle of ethanol injection which are namely cross flow-injection technique [60, 61, 62, 63], microfluidic channel method [64, 65, 66] and membrane contactor method [67].
7.5.1 Cross flow injection method
This cross flow is one of large scalable liposome preparation technique with a module with two tubes welded crossly [60, 61, 62, 63]. To the cross connecting point, one injection hole is adapted. The used concentration of lipid, injecting hole diameter, injection pressure, flowing rate of buffer and performance of system are important parameters to be considered in preparation of liposomes [62]. A minimum amount of buffer flow rate and lipid concentration with higher injection pressures is needed for batch homogeneity. Reproducibility and scale up data of prepared liposomes with this method showed good results on vesicle size, size distribution, stability and robustness [60].
7.5.1.1 Microfluidization
Liposomes are prepared by injecting lipid and water phases with microfluidic hydrodynamic focusing (MHF) into a microchannel [64]. Microfluidic flow is a low rate laminar flow with. Uniform mixing is observed when multi flow steams injected into microchannel [64].
7.5.2 Membrane contactor
In recent studies ethanol injection method was used along with membrane contractor for large production of liposomes [67]. The lipid phase contains cholesterol and phospholipid mixed in ethanol. That mixture has to be pressed through membrane using nitrogen gas with pressure below 5 bars. Tangentially aqueous phase was passed through the same membrane into organic phase to form liposomal dispersion. This new process has advantages with simple design, control of liposome size and scale up batch abilities [67].
8. Recent technologies for preparation of liposomes
Different liposome technologies developed for preparation of liposome formulations. All these technologies have their unique characteristics with unique properties for drug delivery.
8.1 Stealth liposomal technology
In stealth liposomal technology method some strands of polymer are attached to drug molecule for safety to that therapeutic agents. In PEGylation process polyethylene glycol is used. Linkage of PEG to liposomes protects drug molecules in physiochemical properties along with changes in hydrodynamic size and prolongs circulatory time. PEGylation reduces frequency of dosage and provides hydrophilic nature for hydrophobic drugs. Drug efficacy will not be changed by this method and also shows reduced toxicity [68]. With the help of this technology a liposome-based formulation Doxil® which is intravenous injection was prepared for ovarian cancer, multiple myeloma, and Kaposi’s sarcoma associated with HIV.
8.2 Non-PEGylated liposomal technology
Non-PEGylated liposome technology (NPLT) is another technology for liposomes delivery in cancer treatment which has more benefits compared to PEGylation process. This technology eliminates PEG side effects and hand foot syndrome (HFS) in chemotherapy treatment. Non-PEGylated liposome Doxorubicin (NPLD) decreases cardiac toxicity related with DOX and dose limiting toxicity with Doxil® like painful HFS [69]. Myocet® is another NPLD used in advanced stage IV breast cancer which was manufactured by company Elan Pharmaceuticals.
8.3 DepoFoam™ liposome technology
This technology was invented by Pacira Pharmaceuticals for preparation of multivesicular liposomes without changing molecular structure of encapsulated drug and releases drug for long period (1 to 30 days). DepoFoam® is a core technology for liposomal marketed products in names Depocyt(e)® containing cytarabine, DepoDur® contains morphine sulfate and Exparel® contains bupivacaine. These formed vesicles are microscopic spheroids with 3–30 μm in size having granular structure with single layered lipid molecules composed like honeycomb and drug molecules are loaded in central aqueous core [70].
Thermally sensitive liposomes release drug from sites which have elevated temperature in our body. Phospholipids like DPPC and MSPC are generally used for preparing these types of liposomes. They are called temperature dependent liposomes. Lipid components present in these thermosensitive liposomes first forms gel to liquid during higher temperature which will be more permeable for drug release. ThermoDox® liposomal formulation prepared with LTSL technology by Celsion Corporation contains doxorubicin drug and present in clinical phase III trials. ThermoDox® has more drug concentration compared to intravenous doxorubicin and present in clinical studies (phase II) used in breast cancer associated to chest wall [71].
Liposomes are mainly characterized physically, chemically and biologically.
9.1 Physical characterization
The physical characterization of liposomes will be known by evaluating their shape, size, morphology, lamellarity behavior and drug release.
9.1.1 Vesicle shape, size and morphology
The liposomes physical stability depends on their size and polydispersibility index [72]. The size is important in parenteral formulations [73]. Electron microscopy is used for measurement of liposome vesicle size and determining their morphology and lamellarity [74, 75, 76]. Transmission electron microscopy (TEM) is mostly used in measurement of size distribution. Cryo transmission electron microscopy helps in visualizing liposomes that are in frozen state [77]. This is advantageous because analysis is done at their storage environment and prevents disruption of vesicles [78]. Liposomes are applied in a thin film form to a grid and that grid is to be kept in cooling medium (mostly liquid nitrogen) and viewed under microscope and imaged [79, 80]. This microscopy method is advantageous because vesicles can be measured individually which gives detailed information in size and matrix [81, 82].
9.1.2 Surface charge
The surface charge of liposomes in dispersion can be known by zeta potential [82]. The zeta potential is the total charge obtained by liposomes in liposomal dispersion [83]. The liposomes stability always depends on zeta potential [84]. The liposomal dispersion is always stable when vesicles remain separate without any aggregation. When vesicles have charge then repulsion is seen between vesicles in dispersion with repulsive forces and become stable. For stable liposomal dispersion there will be maximum vesicle charge. Liposomal dispersion with zeta potential of greater that 30 mV or lesser than −30 mV are considered to be more stable.
9.1.3 Lamellarity
The lipid bilayers present in liposomal vesicles represents the lamellarity. This lamellarity of liposomes is mostly applicable in encapsulation efficiency, drug release, fate of drug, and applications [85]. Lamellarity is identified using nuclear magnetic resonance (NMR) spectroscopy. In this 31P NMR method, paramagnetic ion Mn2+ or Co2+ or Pr3+ are added into liposomal dispersion [86, 87]. These ions quenches 31P signal from outer part of phospholipids on reaction with negative phosphate groups. This causes disturbance in spin relaxation and decreases 31P resonance signal. The lamellarity is calculated from comparing signal before and after addition of reagent.
9.1.4 Liposomal dispersion phase behavior
Liposomal dispersion phase behavior can be identified by using differential scanning calorimetry (DSC) [88]. Differential scanning calorimetry method depends on temperature measurement at excess heat capacity of liposomes [89].
9.1.5 Encapsulation efficiency
Percentage encapsulation efficiency (% EE) can be determined by ultracentrifugation method [90, 91]. To find out the entrapment efficiency the liposomal dispersions are to be centrifuged at 5° C at 18,000 rpm for 1 h. The sediment portion of the mixture containing liposomes will be separated and lysised using methanol. Then the concentration of drug from lysised liposomes after suitable dilution was estimated by using UV Visible Spectrophotometer at respective wave length. The entrapment efficiency can be calculated by using following formula.
Entrapped Efficiency=Entrapped Drug ContentTotal Drug Content×100E1
9.2 Chemical characterization
Chemical characterization studies gives results for identification of purity in liposomal constituents.
9.2.1 Phospholipids concentration
Phospholipids concentration can be known by using barrlet assay and its principal depends on colorimetric method by inorganic phosphate measurement. The concentration of phospholipid in liposomes is identified by addition of perchloric acid and that gives inorganic phosphate. On adding ammonium molybdate, inorganic phosphate will be converted into phospho-molybdic acid. On adding 4-amino-2-napthyl-4-sulfonic acid to phospho-molybdic acid under hot condition its gives blue color complex which can be determined calorimetrically at 830 nm.
9.2.2 Cholesterol concentration
The adequate separation of cholesterol and its oxidation products in liposomal dispersion can be analyzed by HPLC method. This is mainly studied in stability tests for liposomal formulations.
9.2.3 Fatty acid composition in phospholipids
The fatty acid composition in phospholipid or liposomal dispersion is analyzed by gas chromatography. This method is suitable in estimation of fatty acids oxidation. Two types of column are used in gas chromatography. One is packed column in which liquid phase is coated on granular support and packed into a coiled tube of glass or stainless steel. Other is capillary column which is much narrower in bore, longer and made of glass or fused silica capillary and contains no packing but the liquid phase is coated directly on to inner capillary wall.
9.2.4 Phospholipids per-oxidation
Most oxidation products are further subjected to degradation and at least two separate tests should be performed for estimation of oxidation. Gas liquid chromatography (GLC) and UV absorbance are most quantitative methods to estimate oxidation. This UV method is based on the absorbance of conjugated dienes and trienes at 233 nm 270 nm and phospholipids do not absorb at these wave lengths. TBA (thiobarbituric acid) method is widely used lipid peroxidation assay. In this method the samples are heated with an aqueous TBA solution. Under these conditions the lipid oxidation product malondialdehyde reacts with TBA gives a pink chromophore and spectrophotometrically quantified at 533 nm.
9.2.5 Phospholipids hydrolysis
The phospholipids in liposomes will hydrolyse to free fatty acids and 2-acyl- and 1-acyl-lysophospholipids. The lysophospholipids further hydrolysed to glycerol phosphor compounds. The hydrolysed products can be analyzed by using HPLC method or TLC method and glycerol phosphor compounds can be analyzed by total phosphate analysis of the supernatant (methanol/water phase) after lipid extraction.
9.3 Biological characterization
Biological characterization identifies safety of liposomes formulations when in vivo studies are done [16]. The liposomes characterization depends on selection of phospholipid, size characteristics and charge behavior [92, 93]. Sterility of liposomes can be identified by preparing aerobic or anaerobic cultures and pyrogenicity can be known by pyrogen test on rabbits.
Stability is that prescribed preparation should remain with required pre-established limits at predetermined time period. Chemical instability is physical not stability with leakage in loaded drug molecules from lipid bilayers, vesicles fusion with forming aggregation [94]. Physicochemical instability of liposome suspension like hydrolysis, aggregation, fusion and oxidation can be avoided by preparing pro-liposomes [95]. Liposomal stability will be increased when preparing efficient formulation using lyophillization or freeze drying to form powder liposomes which can be reconstituted upon usage. Selection of lipid composition, concentration of bilayers, buffers, antioxidant, chelating agents and cryo protectants play vital role in liposomes preparation. Buffer solutions having neutral pH decreases hydrolysis and antioxidant like sodium ascorbate will decrease oxidation in liposomal dispersion. Oxygen potential is minimized with nitrogen gas purging to liposomal dispersion to obtain stable formulation [96]. Using antioxidants, buffers with neutral pH and lyo or cryo protectants in freeze drying also gives stable liposomal formulation.
11. Applications of liposomes
Liposomes with varying size, morphology, lipid composition and cholesterol are suitable for many applications to drug delivery [97]. Liposome vesicles interact with body cells for targeted drug delivery [98].
11.1 Liposome for respiratory disorders
Liposome formulations are used in lung disorders and respiratory aerosol which contains liposomes has more advantage compared to normal aerosol. Liposomal aerosols have advantages like sustained action, no local irritation, less toxicity and more stability [93]. Liposome products related to respiratory disorders available in market with brand names are ambisome, Myocet and Fungisome.
11.2 Liposome in nucleic acid therapy
The liposomes bind to nucleic acid with passive charge lipids and pH related surfactants [99, 100]. Liposomes related to gene delivery are under research [101, 102].
11.3 Liposome in eye disorders
Liposomes are used with disorders associated to front and later segments. Retinal diseases cause eye blindness in most advanced countries. Liposomal formulations related to eye disorders are approved for patents and few are in clinical trials. Verteporfin is one of the liposomal based products used in leak of blood vessels at eye caused by pathologic myopia, histoplasmosis (a fungal infection) to eye and age-related macular degeneration.
11.4 Vaccine adjuvant liposomes
Liposomes acts as immune adjuvant and has potentiating activity for cell and non cell mediated immunity [103]. Liposomes are immunological (vaccine) adjuvant, vaccines, carrier of immune modulation and tool in immune diagnostics. Liposomal immuno-adjuvant releases encapsulated antigen very slowy and passively accumulates in lymph node [104] by coupling to the liposomal membrane [105]. Liposomal vaccines can be stored in refrigerator for about 12 months.
11.5 Brain targeting liposomes
Liposomes have biocompatible and eco-friendly action which leads to exploration to brain drug delivery [106]. Liposomes with small and large size undergo easy diffusion through blood brain barrier (BBB). Small unilamelar vesicles (SUVs) coupled to brain drug transport maybe transported through blood brain barrier by receptors or transcytosis. To the liposomes preparations on addition of sulphatide will help to cross blood brain barrier [107]. Wang et al. reported in his studies that liposomes with mannose coated targets brain tissue by passing through blood brain barrier [108, 109].
11.6 Liposome as anti-infective agents
Amphotericin B (ambisome) is now available in liposome based formulation has passed all the clinical trials. Liposomal Amphotericin B targets liver and spleen by passively and reduces renal toxicity at normal dose and toxicity appears back when given at higher dose [110, 111].
11.7 Liposome in tumor therapy
Long therapy treatment with anticancer drugs gives several toxic effects but liposomal for tumor cells showed less side effects. Reported methods showed liposomes targets to tumor cells and circulate for longer time period with enhanced vascular permeability [112, 113]. In the year 1995, Doxil which is Doxorubicin PEGylated liposomes, for intravenous administration prepared by stealth technology was approved for hematological tumors [5, 6]. Caelyx and myocet are other liposome preparations for same doxorubicin used for advanced breast cancer [114, 115, 116].
12. Commercially and clinically available liposomal based products
Various attempts have been made in research to develop novel liposomal formulations for commercial importance and some are under clinical trials. Liposomes have gained their commercial importance with Intravenous formulations. However some recently developed liposomal formulations which are under clinical trials is Arikace (Amikacin for lung infections) [117, 118] which can be given as subcutaneous injection or inhaled as aerosols. Apart from intravenous route and nasal route, the research is being focused and investigated on liposomal formulations for topical route by applying new strategies in the preparation of cosmetics such as skin creams, anti-aging creams, after shave, lipstic, sun screen and make-up [119]. The liposomal-based drugs that are available in market and are under clinical trials are shown in Tables 1 and 2 respectively.
In the last decade, liposomes have become much popular due to research and commercial importance. They offer several advantages for the delivery of different molecules in various routes of administration. Hence the regulatory agencies through the globe is also implemented the subject of liposome into their guidelines. In conclusion, continuous efforts are going in the area of liposome technology to make them better for drug delivery.
Acknowledgments
The authors are thankful to University Grants Commission (UGC, New Delhi, India) for providing financial assistance for carrying the research work. The authors would like to acknowledge M/s. GITAM Institute of Pharmacy, Gandhi Institute of Technology and Management (GITAM) Deemed to be University, Rushikonda, Visakhapatnam, Andhra Pradesh, India for providing facilities and giving support to conduct this work.
Conflict of interest
There is no conflict of interest.
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Written By
Srinivas Lankalapalli and V.S. Vinai Kumar Tenneti
Submitted: 04 March 2021Reviewed: 14 April 2021Published: 06 July 2022
Open access peer-reviewed chapter
