NIOSOME ( as a drug carrier)
1.1. Historical aspect-
Niosomes were first reported in the seventies as a feature of the cosmetic industry by Vanlerberghe et al, Handjani-vila et al., Van Abbe
explained that the non – inonic surfactants are preferred because the irritation power of surfactants decreases in the following order: cationic > anionic > ampholytic > non-ionic. Green and Downs, Keller et al., Burstein, Kaur and Smitha reported that an increased ocular bioavailability of water soluble, entrapped in niosomes, may be due to the fact that surfactants also act as penetration enhancers as they can remove the mucus layer and break functional complexes. Handjani-Vila et al. reported that vesicular systems were formed when a mixture of cholesterol and single alkyl chain, non-ionic surfactants was hydrated. The resultant vesicles, termed as niosomes can entrap solute. Okhata et al. suggested vesicle formation by some members of dialkylpoly oxyethlene ether non-ionic surfactant series. Singh and Mezei stated that niosomes are a suitable delivery system for both hydrophilic and lipophilic drugs. Baillie et al.
reported that niosomes are osmotically active and relatively stable. Lasic
stated that the assembly into closed bilayers is rarely spontaneous and usually involves some input of energy such as physical agitation or heat. The result is an assembly in which the hydrophobic parts of the molecule are shielded from the aqueous solvent and the hydrophilic head groups enjoy maximum contact with same. The non-ionic surfactant vesicles have been reported successfully by Saettone et al. as ocular vehicle for cyclopentolate. Carafa et al. stated that niosomes are the non-ionic surfactant vesicles and like liposomes are bilayered structures, which can entrap both hydrophilic and lipophillic drugs either in an aqueous layer or in vesicular membrane, made up of lipids. Vyas et al.
prepared both niosomes and discomes of water-soluble drug timolol maleate and found that discomes entrapped comparatively a higher amount of drug (25% as compared to 14% in case of niosomes). Moreover, an increase in ocular bioavailability was found to be approximately 3.07-fold compared to 2.48-fold in case of niosomes with respect to timolol maleate solution. Carafe et al.
reported that niosomes are biodegradable, biocompatible, and non-immunogenic. Indu P. Kaur et al. gave an impression on vesicular system in ocular drug delivery. Deepika Aggarwal et al. studied the improved pharmacodynamics of timolol maleate from a mucoadhesive niosomal ophthalmic drug delivery system. Deepika Aggarwal et al. studied the ocular absorption of acetazolamide by microdialysis sampling of aqueous humor. Ghada abdelbary and Nashwa el-gendy investigated the feasibility of using non-ionic surfactant vesicles as carriers for the ophthalmic controlled delivery of a water soluble local antibiotic, Gentamicin sulphate.
At present no available drug delivery system achieves the site specific delivery with controlled release kinetics of drug in predictable manner.
Paul Ehrlich, in 1909, initiated the era of development for targeted delivery when he envisaged a drug delivery mechanism that would target directly to diseased cell. Since then, number of carriers was utilized to carry drug at the target organ/tissue, which include immunoglobulins, serum proteins, synthetic polymers, liposomes, microspheres, erythrocytes, niosomes etc.
Among different carriers liposomes and niosomes are well documented drug delivery.
Drug targeting can be defined as the ability to direct a therapeutic agent specifically to desired site of action with little or no interaction with nontarget tissue.
Niosomes or non-ionic surfactant vesicles are microscopic lamellar structures formed on admixture of non-ionic surfactant of the alkyl or dialkyl polyglycerol ether class and cholesterol with subsequent hydration in aqueous media.
In niosomes, the vesicles forming amphiphile is a non-ionic surfactant such as Span – 60 which is usually stabilized by addition of cholesterol and small amount of anionic surfactant such as dicetyl phosphate.
Niosomes are non-ionic surfactant vesicles obtained on hydration of synthetic nonionic surfactants, with or without incorporation of cholesterol or other lipids.
They are vesicular systems similar to liposomes that can be used as carriers of amphiphilic and lipophilic drugs. Niosomes are promising vehicle for drug delivery and being non-ionic; it is less toxic and improves the therapeutic index of drug by restricting its action to target cells. This systemic review article deals with preparation methods, characterizations, factors affecting release kinetic, advantages, and applications of niosomes. Schematic representation of a drug targeting through its linkage to niosome via antibody is shown in figure 1-
Figure 1: Niosome structure
Figure 2: Niosome structure
1.3. Advantages of niosomes:-
- They entrap solute in a manner analogous to liposomes.
- They are osmotically active and stable.
- Handling and storage of surfactants requires no special conditions.
- They possess an infrastructure consisting of hydrophobic and hydrophilic moieties together and as a result can accommodate drug molecules with a wide range of solubilities.
- They exhibit flexibility in their structural characteristics (composition, fluidity, and size) and can be designed according to desired application.
- They improve oral bioavailability of poorly absorbed drugs and enhance skin penetration of drugs.
- They allow their surface for attachment of hydrophilic group and can incorporate hydrophilic moieties in bilayer to bring about changes in their in vivo behavior.
- The surfactants are biodegradable, biocompatible, and nonimmunogenic.
- They improve the therapeutic performance of the drug molecules by delaying the clearance from the circulation, protecting the drug from biological environment, and restricting effects to target cells.
- Niosomal dispersion in an aqueous phase can be emulsified in a nonaqueous phase to regulate the delivery rate of drug and administer normal vesicle in external nonaqueous phase.
2.1. Small unilamellar vesicles- (SUV, size -0.025-0.05 μm) are commonly produced by sonication, and French Press procedures. Ultrasonic electrocapillary emulsification or solvent dilution techniques can be used to prepare SUVs.
2.2. Multilamellar vesicles- (MLV, size >0.05 μm) exhibit increased-trapped volume and equilibrium solute distribution, and require hand-shaking method. They show variations in lipid compositions.
2.3. Large unilamellar vesicles- (LUV, size >0.10 μm), the injections of lipids solubilised in an organic solvent into an aqueous buffer, can result in spontaneous formation of LUV. But the better method of preparation of LUV is Reverse phase evaporation, or by Detergent solubilisation method.
3. Factors affecting formation of niosomes:-
Entrapment of drug in niosomes increases vesicle size, probably by interaction of solute with surfactant head groups, increasing the charge and mutual repulsion of the surfactant bilayers, thereby increasing vesicle size. In polyoxyethylene glycol (PEG) coated vesicles; some drug is entrapped in the long PEG chains, thus reducing the tendency to increase the size. The hydrophilic lipophilic balance of the drug affects degree of entrapment.
b) Amount and type of surfactant-
The mean size of niosomes increases proportionally with increase in the HLB of surfactants like Span 85 (HLB 1.8) to Span 20 (HLB 8.6) because the surface free energy decreases with an increase in hydrophobicity of surfactant.
The bilayers of the vesicles are either in the so-called liquid state or in gel state, depending on the temperature, the type of lipid or surfactant and the presence of other components such as cholesterol. In the gel state, alkyl chains are present in a well-ordered structure, and in the liquid state, the structure of the bilayers is more disordered. The surfactants and lipids are characterized by the gel-liquid phase transition temperature (TC). Phase transition temperature (TC) of surfactant also effects entrapment efficiency i.e. Span 60 having higher TC, provides better entrapment.
c) Cholesterol content and charge-
Inclusion of cholesterol in niosomes increases its hydrodynamic diameter and entrapment efficiency. In general, the action of cholesterol is two folds; on one hand, cholesterol increases the chain order of liquid-state bilayers and on the other, cholesterol decreases the chain order of gel state bilayers. At a high cholesterol concentration, the gel state is transformed to a liquid-ordered phase.
An increase in cholesterol content of the bilayers resulted in a decrease in the release rate of encapsulated material and therefore an increase of the rigidity of the bilayers obtained. Presence of charge tends to increase the interlamellar distance between successive bilayers in multilamellar vesicle structure and leads to greater overall entrapped volume.
d) Methods of preparation-
Methods of preparation of niosomes such as hand shaking, ether injection and sonication have been reviewed by Khandare et al. Hand shaking method forms vesicles with greater diameter (0.35-13nm)
compared to the ether injection method (50-1000nm).
Small sized niosomes can be produced by Reverse Phase Evaporation (REV) method .Microfluidization method gives greater uniformity and small size vesicles. Parthasarthi et al prepared niosomes by Trans membrane pH gradient (inside acidic) drug uptake process. Niosomes obtained by this method showed greater entrapment efficiency and better retention of drug.
e.) Nature of surfactants-
A surfactant used for preparation of niosomes must have a hydrophilic head and hydrophobic tail. The hydrophobic tail may consist of one or two alkyl or perfluoroalkyl groups or in some cases a single steroidal group. The ether type surfactants with single chain alkyl as hydrophobic tail is more toxic than corresponding dialkylether chain. The ester type surfactants are chemically less stable than ether type surfactants and the former is less toxic than the latter due to ester-linked surfactant degraded by esterases to triglycerides and fatty acid in vivo. The surfactants with alkyl chain length from C12-C18 are suitable for preparation of niosome. Surfactants such as C16EO5 (poly-oxyethylene cetyl ether) or C18EO5 (polyoxyethylenesteryl ether) are used for preparation ofpolyhedral vesicles.Span series surfactants having HLB number of between 4 and 8 canform vesicles.
f.) Structure of surfactants-
The geometry of vesicle to be formed from surfactants isaffected by its structure, which is relatedto critical packingparameters. On the basis of critical packing parameters ofsurfactants canpredicate geometry of vesicle to be formed.Critical packing parameters can be defined usingfollowingequation,
Where v = hydrophobic group volume,
lc = the critical hydrophobic group length,
a0 = the area of hydrophilic head group.
From the critical packing parameter value type ofmiceller structure formed can be ascertained as given below,
If CPP < ½ then formation of spherical micelles,
If ½ < CPP < 1 formation of bilayer micelles, &
If CPP > 1 formation inverted micelles.
g.) Membrane composition-
The stable niosomes can be prepared with addition ofdifferent additives along with surfactants and drugs.Niosomes formed have a number of morphologies andtheir permeability and stability properties can be altered bymanipulating membrane characteristics by differentadditives. In case of polyhedral niosomes formed from C16G2, the shape of these polyhedral niosome remains
unaffected by adding low amount of solulan C24 (cholesteryl poly-24-oxyethylene ether), which preventsaggregation due to development of steric hindrance. In contrast sphericalniosomes are formed by C16G2: cholesterol: solulan (49:49:2) 2000).The mean size ofniosomes is influenced by membrane composition such as polyhedral niosomes formed by C16G2: solulan C24 in ratio(91:9) having bigger size (8.0 ± 0.03mm) thanspherical/tubular niosomes formed by C16G2:cholesterol:solulan C24 in ratio (49:49:2) Addition of cholesterolmolecule to niosomal system provides rigidity to themembrane and reduces the leakage of drug from niosome.
h.) Nature of encapsulated drug-
The physico-chemical properties of encapsulated druginfluence charge and rigidity of the niosome bilayer. Thedrug interacts with surfactant head groups and develops thecharge that creates mutual repulsion between surfactantbilayers and hence increases vesicle size (Stafford S et al., 1988). The aggregation of vesicles is prevented due to the charge development on bilayer.
i.) Temperature of hydration-
Hydration temperature influences the shape and size of thenoisome. For ideal condition it should be above the gel to liquid phase transition temperature of system. Temperaturechange of niosomal system affects assembly of surfactantsinto vesicles and also induces vesicle shape transformation. Arunothayanun et al. reported that a polyhedral vesicleformed by C16G2: solulan C24 (91:9) at 25°C which on heating transformed into spherical vesicle at 48°C, but oncooling from 55°C, the vesicle produced a cluster ofsmaller spherical niosomes at 49°C before changing to the polyhedral structures at 35°C. In contrast vesicle formed by C16G2: cholesterol:solulanC24(49:49:2) shows no shape transformation onheating or cooling Along with the above mentioned factors, volume of hydration medium and time of hydration of niosomes arealso critical factors. Improper selection of these factorsmay results in formation of fragile niosomes or creation ofdrug leakage problems.
j.) Type of Surfactants-
Type of the surfactants influences encapsulation efficiency, toxicity, and stability of niosomes. The first niosomes were formulated using cholesterol and single-chain surfactants such as alkyl oxyethylenes. The alkyl group chain length is usually from C12–C18. The hydrophilic- lipophilic balance (HLB) is a good indicator of the vesicle forming ability of any surfactant. Uchegbu et al reported that the sorbitan monostearate (Span) surfactants with HLB values between 4 and 8 were found to be compatible with vesicle formation. Polyglycerol monoalkyl ethers and polyoxylate analogues are the most widely used single-chain surfactants.However, it must be noted that they possess less encapsulation efficiency in the presence of cholesterol. Etheric surfactants have also been used to form niosomes. These types of surfactants are composed of single-chain, monoalkyl or dialkyl chain. The latest ones are similar to phospholipids and possess higher encapsulation efficiency. Esther type amphyphilic surfactants are also used forniosome formulation. They are degraded by estherases, triglycerides and fatty acids. Although these types of surfactants are less stable than ether type ones, they possess less toxicity. Furthermore, glucosides of myristil, cethyl and stearyl alcohols formniosomes.
k.) Surfactant/Lipid and Surfactant/Water Ratios-
Other important parameters are the level of surfactant/lipid and the surfactant/water ratio. The surfactant/lipid ratio is generally 10–30 mM (1–2.5% w/w). If the level of surfactant/lipid is too high, increasing the surfactant/lipid level increases thetotal amount of drug encapsulated. Change in the surfactant/water ratio during the hydration process may affect the system's microstructure and thus, the system's properties.
l.) Other Additives-
As is the case with liposomes, charged phospholipids such as dicethylphosphate (DCP) and stearyl amine (SA) have been used to produce Charge in niosome formulations. The former molecule provides negative charge to vesicles whereas the later one is used in the preparation of positively charged (cationic) niosomes.
m.) Nature of the Drug-
One of the overlooked factors is the influence of the nature of the encapsulated drug on vesicle formation (Table 1). The encapsulation of the amphipathic drug doxorubicin has been shown to alter the electrophoretic mobility of hexadecyl diglycerol ether (C16G2) niosomes in a pH dependent manner, indicating that the amphipathic drug is incorporated in the vesicle membrane.
n.) Resistance to osmotic stress-
Addition of a hypertonic salt solution to a suspension of niosomes brings about reduction in diameter. In hypotonic salt solution, there is initial slowrelease with slight swelling of vesicles probably due to inhibition of eluting fluid from vesicles, followed by faster release, which may be due to mechanical loosening of vesicles structure under osmotic stress.
4. Method of prepration:-
a.) Ether injection method-
This method provides a means of making niosomes by slowly introducing a solution of surfactant dissolved in diethyl ether into warm water maintained at 60°C. The surfactant mixture in ether is injected through 14-gauge needle into an aqueous solution of material. Vaporization of ether leads to formation of single layered vesicles. Depending upon the conditions used, the diameter of the vesicle range from 50 to 1000 nm.
b.) Hand shaking method (Thin film hydration technique)-
The mixture of vesicles forming ingredients like surfactant and cholesterol are dissolved in a volatile organic solvent (diethyl ether, chloroform or methanol) in a round bottom flask. The organic solvent is removed at room temperature (20°C) using rotary evaporator leaving a thin layer of solid mixture deposited on the wall of the flask. The dried surfactant film can be rehydrated with aqueous phase at 0-60°C with gentle agitation. This process forms typical multilamellar niosomes. Thermosensitive niosomes were prepared by Raja Naresh et al by evaporating the organic solvent at 60°C and leaving a thin film of lipid on the wall of rotary flash evaporator. The aqueous phase containing drug was added slowly with intermittent shaking of flask at room temperature followed by sonication.
A typical method of production of the vesicles is by sonication of solution as described by Cable . In this method an aliquot of drug solution in buffer is added to the surfactant/cholesterol mixture in a 10-ml glass vial. The mixture is probe sonicated at 60°C for 3 minutes using a sonicator with a titanium probe to yield niosomes.
d.) Micro fluidization-
Micro fluidization is a recent technique used to prepare unilamellar vesicles of defined size distribution. This method is based on submerged jet principle in which two fluidized streams interact at ultra high velocities, in precisely defined micro channels within the interaction chamber. The impingement of thin liquid sheet along a common front is arranged such that the energy supplied to the system remains within the area of niosomes formation. The result is a greater uniformity, smaller size and better reproducibility of niosomes formed.
e.) Multiple membrane extrusion method-
Mixture of surfactant, cholesterol and dicetyl phosphate in chloroform is made into thin film by evaporation. The film is hydrated with aqueous drug polycarbonate membranes,
solution and the resultant suspension extruded through which are placed in series for upto 8 passages. It is a good method for controlling niosome size.
solution and the resultant suspension extruded through which are placed in series for upto 8 passages. It is a good method for controlling niosome size.
f.) Reverse Phase Evaporation Technique (REV)-
Cholesterol and surfactant (1:1) are dissolved in a mixture of ether and chloroform. An aqueous phase containing drug is added to this and the resulting two phases are sonicated at 4-5°C. The clear gel formed is further sonicated after the addition of a small amount of phosphate buffered saline (PBS). The organic phase is removed at 40°C under low pressure. The resulting viscous niosome suspension is diluted with PBS and heated on a water bath at 60°C for 10 min to yield niosomes.
Raja Naresh et al have reported the preparation of Diclofenac Sodium niosomes using Tween 85 by this method.
g.)Trans membrane pH gradient (inside acidic) Drug Uptake Process (remote Loading)-
Surfactant and cholesterol are dissolved in chloroform. The solvent is then evaporated under reduced pressure to get a thin film on the wall of the round bottom flask. The film is hydrated with 300 mM citric acid (pH 4.0) by vortex mixing. The multilamellar vesicles are frozen and thawed 3 times and later sonicated. To this niosomal suspension, aqueous solution containing 10 mg/ml of drug is added and vortexed. The pH of the sample is then raised to 7.0-7.2 with 1M disodium phosphate. This mixture is later heated at 60°C for 10 minutes to give niosomes.
h.) The "Bubble" Method-
It is novel technique for the one step preparation of liposomes and niosomes without the use of organic solvents. The bubbling unit consists of round-bottomed flask with three necks positioned in water bath to control the temperature. Water-cooled reflux and thermometer is positioned in the first and second neck and nitrogen supply through the third neck. Cholesterol and surfactant are dispersed together in this buffer (pH 7.4) at 70°C, the dispersion mixed for 15 seconds with high shear homogenizer and immediately afterwards "bubbled" at 70°C using nitrogen gas.
i.) Formation of niosomes from proniosomes-
Another method of producing niosomes is to coat a water-soluble carrier such as sorbitol with surfactant. The result of the coating process is a dry formulation. In which each water-soluble particle is covered with a thin film of dry surfactant. This preparation is termed "Proniosomes". The niosomes are recognized by the addition of aqueous phase at T > Tm and brief agitation.
T = Temperature.
Tm = mean phase transition temperature.
Tm = mean phase transition temperature.
Blazek-Walsh A.I. et al have reported the formulation of niosomes from maltodextrin based proniosomes. This provides rapid reconstitution of niosomes with minimal residual carrier. Slurry of maltodextrin and surfactant was dried to form a free flowing powder, which could be rehydrated by addition of warm water.
Table 1: Drugs incorporated into niosomes by various methods-
|Method of preparation||Drug incorporated|
|Ether Injection||Sodium stibogluconate |
|Hand Shaking||Methotrexate |
The term surfactant is a blend of surface active agent. Surfactants are usuallyorganiccompounds that are amphiphilic, meaning they contain both hydrophobic groups (their tails) and hydrophilic groups (their heads). Therefore, a surfactant molecule contains both a water insoluble (and oil soluble component) and a water soluble component. Surfactant molecules will migrate to the water surface, where the insoluble hydrophobic group may extend out of the bulk water phase, either into the air or, if water is mixed with oil, into the oil phase, while the water soluble head group remains in the water phase. This alignment and aggregation of surfactant molecules at the surface, acts to alter the surface properties of water at the water/air or water/oil interface.
Cetyl alcohol, Stearyl alcohol, Cetostearyl alcohol (consisting predominantly of cetyl and stearyl alcohols), Oleyl alcohol;
Polyoxyethylene glycol alkyl ethers (Brij): CH3–(CH2)10–16–(O-C2H4)1–25–OH:
Octaethylene glycol monododecyl ether, Pentaethylene glycol monododecyl ether;
Polyoxypropylene glycol alkyl ethers: CH3–(CH2)10–16–(O-C3H6)1–25–OH;
Glucoside alkyl ethers: CH3–(CH2)10–16–(O-Glucoside)1–3–OH: eg. Decyl glucoside, Lauryl glucoside, Octyl glucoside;
Polyoxyethylene glycol octylphenol ethers: C8H17–(C6H4)–(O-C2H4)1–25–OH: eg.Triton X-100;
Polyoxyethylene glycol alkylphenol ethers:C9H19–(C6H4)–(O-C2H4)1–25–OH: eg.Nonoxynol-9;
Glycerol alkyl esters: eg.Glyceryl laurate
Polyoxyethylene glycol sorbitan alkyl esters: eg.Polysorbates;
Sorbitan alkyl esters: eg.
Block copolymers of polyethylene glycol and polypropylene glycol: eg.Poloxamers
Steroids are important components of cell membranes and their presence in membranes brings about significant changes with regard to bilayer stability, fluidityand permeability. Cholesterol, a natural steroid, is the most commonly usedmembrane additive and can be incorporated to bilayers at high molar ratios. Cholesterol by itself, however, does not form bilayer vesicles. It is usuallyincluded in a 1:1 molar ratio in most formulations to prevent vesicle aggregationby the inclusion of molecules that stabilize the system against the formation ofaggregates by repulsive steric or electrostatic effects. It leads to the transition fromthe gel state to liquid phase in niosome systems. As a result, niosomes become lessleaky.
Entrapment of drug in niosomes increases vesicle size, probably by interaction of solute withsurfactant head groups, increasing the charge and mutual repulsion of the surfactant bilayers, thereby increasing vesicle size. In polyoxyethylene glycol (PEG) coated vesicles, some drug is entrapped in the long PEG chains, thus reducing the tendency to increase the size. The hydrophilic lipophilic balance of the drug affects degree of entrapment
d.) Other Additives-
As is the case with liposomes, charged phospholipids such as dicethylphosphate
(DCP) and stearyl amine (SA) have been used to produce charge in niosome formulations. The former molecule provides negative charge to vesicles whereas the later one is used in the preparation of positively charged (cationic) niosomes.
5.Evaluation parameters or Characterization of niosome:-
5.1.Separation of Unentrapped Drug:-
The removal of unentrapped solute from the vesicles can be accomplished by various techniques, which include: -
The aqueous niosomal dispersion is dialyzed in dialysis tubing against phosphate buffer or normal saline or glucose solution.
2. Gel Filtration-
The unentrapped drug is removed by gel filtration of niosomal dispersion through a Sephadex-G-50 column and elution with phosphate buffered saline or normal saline.
3. Centrifugation-The niosomal suspension is centrifuged and the supernatant is separated. The pellet is washed and then resuspended to obtain a niosomal suspension free from unentrapped drug
5.2.Evaluation parameters or Characterization of niosome-
Shape of niosome vesicles assumed to be spherical, their mean diameter can be determined by using laser light scattering method . Also, diameter can be determined by using electron microscopy, molecular sieve chromatography, ultracentrifugation, photon correlation microscopy and optical microscopy
b.) Bilayer formation-
Assembly of non-ionic surfactants to form bilayer vesicle is characterized by X-cross formation under light polarization microscopy.
c.) Number of lamellae-
It is determined by using NMR spectroscopy, small angle X-ray scattering and electron microscopy
d.) Membrane rigidity-
Membrane rigidity can be measured by means of mobility of fluorescence probe as function of temperature
e.) Entrapment efficiency-
After preparing niosomal dispersion, unentrapped drug is separated by dialysis centrifugation, or gel filtration as described above and the drug remained entrapped in niosomes is determined by complete vesicle disruption using 50% n-propanol or 0.1% Triton X-100 and analysing the resultant solution by appropriate assay method for the drug. Where,
Percent (%) entrapped efficiency = [Entrapped drug (mg) / Total drug added (mg)] X 100
f.) Vesicle diameter-
Niosomes, similar to liposomes, assume spherical shape and so their diameter can be determined using light microscopy, photon correlation microscopy and freeze fracture electron microscopy. Freeze thawing (keeping vesicles suspension at –20°C for 24 hrs and then heating to ambient temperature) of niosomes increases the vesicle diameter, which might be attributed to fusion of vesicles during the cycle.
g.) In-vitro release-
A method of in-vitro release rate study includes the use of dialysis tubing. A dialysis sac is washed and soaked in distilled water. The vesicle suspension is pipetted into a bag made up of the tubing and sealed. The bag containing the vesicles is placed in 200 ml of buffer solution in a 250 ml beaker with constant shaking at 25°C or 37°C. At various time intervals, the buffer is analyzed for the drug content by an appropriate assay method.h.) Stability of niosomes-
Vesicles are stabilized based upon formation of 4 different forces:
1. Van der Waals forces among surfactant molecules;
2. Repulsive forces emerging from the electrostatic interactions among charged groups of surfactant molecules;
3. Entropic repulsive forces of the head groups of surfactants;
4. short-acting repulsive forces.
Electrostatic repulsive forces are formed among vesicles upon addition of charged
Surfactants to the double layer, enhancing the stability of the system. Biological stability of the niosomes prepared with alkyl glycosides was investigated by Kiwada et al. They reported that niosomes were not stable enough in plasma. This may be due to single–chain alkyl surfactants. SUVs were found to be more stable. Niosomes in the form of liquid crystal and gel can remain stable at both room temperature and 4°C for 2 months. No significant difference has been observed between the stability of these two types of niosomes with respect to leakage. Even though no correlation between storage temperature and stability has been found, it is recommended that niosomes should be stored at 4°C. Ideally these systems should be stored dry for reconstitution by nursing staff or by the patient and when rehydrated should exhibit dispersion characteristics that are similar to the original dispersion. Simulation studies conducted to investigate physical stability of these niosomes during transportation to the end-user revealed that mechanical forces didn't have anyinfluence on physical stability. It is assumed that the reason behind the stability of niosomes may be due to the prevention of aggregation caused by steric interactions among large polar head groups of surfactants.
The factors which affect the stability of niosomes are as following:
• Type of surfactant;
• Nature of encapsulated drug;
• Storage temperature;
• Use of membrane spanning lipids;
• The interfacial polymerization of surfactant monomers in situ;
• Inclusion of a charged molecule.
i.) Toxicity of niosomes-
Unfortunately, there is not enough research conducted to investigate toxicity of niosomes. Researchers measured proliferation of keratinocytes in one of the topical niosome formulations. The effect of surfactant type on toxicity was investigated. It was determined that the ester type surfactants are less toxic than ether type surfactants. This may be due to enzymatic degradation of ester bounds. In general, the physical form of niosomes did not influence their toxicity as evident in a study comparing the formulations prepared in the form of liquid crystals and gels. However, nasal applications of these formulations caused toxicity in the case of liquid crystal type niosomes. In some instances, encapsulation of the drug by niosomes reduces the toxicity as demonstrated in the study on preparation of niosomes containing vincristine. It decreased the neurological toxicity, diarrhoea and alopecia following the intravenous administration of vincristineand increased vincristine anti-tumor activity in S-180 sarcoma and Erlich ascites mouse models.
Niosomal drug delivery is potentially applicable to many pharmacological agents for their action against various diseases. Some of their therapeutic applications are discussed below.
1) Targeting of bioactive agents-
a) To reticulo-endothelial system (RES)
The cells of RES preferentially take up the vesicles. The uptake of niosomes by the cells is also by circulating serum factors known as opsonins, which mark them for clearance. Such localized drug accumulation has, however, been exploited in treatment of animal tumors known to metastasize to the liver and spleen and in parasitic infestation of liver.
b) To organs other than RES
It has been suggested that carrier system can be directed to specific sites in the body by use of antibodies. Immunoglobulins seem to bind quite readily to the lipid surface, thus offering a convenient means for targeting of drug carrier. Many cells possess the intrinsic ability to recognize and bind particular carbohydrate determinants and this can be exploited to direct carriers system to particular cells.
Doxorubicin, the anthracyclic antibiotic with broad spectrum anti tumor activity, shows a dose dependant irreversible cardio toxic effect. Niosomal delivery of this drug to mice bearing S-180 tumor increased their life span and decreased the rate of proliferation of sarcoma. Niosomal entrapment increased the half-life of the drug, prolonged its circulation and altered its metabolism. Intravenous administration of methotrexate entrapped in niosomes to S-180 tumor bearing mice resulted in total regression of tumor and also higher plasma level and slower elimination.
Niosomes can be used for targeting of drug in the treatment of diseases in which the infecting organism resides in the organ of reticulo-endothelial system. Leishmaniasis is such a disease in which parasite invades cells of liver and spleen. The commonly prescribed drugs are antimonials, which are related to arsenic, and at high concentration they damage the heart, liver and kidney.
The study of antimony distribution in mice, performed by Hunter et al showed high liver level after intravenous administration of the carriers' forms of the drug.
Baillie et al reported increased sodium stibogluconate efficacy of niosomal formulation and that the effect of two doses given on successive days was additive.
4) Delivery of peptide drugs-
Yoshida et al investigated oral delivery of 9-desglycinamide, 8-arginine vasopressin entrapped in niosomes in an in-vitro intestinal loop model and reported that stability of peptide increased significantly.
5) Immunological application of niosomes-
Niosomes have been used for studying the nature of the immune response provoked by antigens. Brewer and Alexander have reported niosomes as potent adjuvant in terms of immunological selectivity, low toxicity and stability.
6) Niosomes as carriers for Hemoglobin-
Niosomes can be used as a carrier for hemoglobin. Niosomal suspension shows a visible spectrum superimposable onto that of free hemoglobin. Vesicles are permeable to oxygen and hemoglobin dissociation curve can be modified similarly to non-encapsulated hemoglobin.
7) Transdermal delivery of drugs by niosomes-
Slow penetration of drug through skin is the major drawback of transdermal route of delivery. An increase in the penetration rate has been achieved by transdermal delivery of drug incorporated in niosomes. Jayraman et al has studied the topical delivery of erythromycin from various formulations including niosomes or hairless mouse. From the studies, and confocal microscopy, it was seen that non-ionic vesicles could be formulated to target pilosebaceous glands.
8) Other Applications-
a) Sustained Release
Azmin et al suggested the role of liver as a depot for methotrexate after niosomes are taken up by the liver cells. Sustained release action of niosomes can be applied to drugs with low therapeutic index and low water solubility since those could be maintained in the circulation via niosomal encapsulation.
b) Localized Drug Action
Drug delivery through niosomes is one of the approaches to achieve localized drug action, since their size and low penetrability through epithelium and connective tissue keeps the drug localized at the site of administration.
Localized drug action results in enhancement of efficacy of potency of the drug and at the same time reduces its systemic toxic effects e.g. Antimonials encapsulated within niosomes are taken up by mononuclear cells resulting in localization of drug, increase in potency and hence decrease both in dose and toxicity.
The evolution of niosomal drug delivery technology is still at an infancy stage, but this type of drug delivery system has shown promise in cancer chemotherapy and anti-leishmanial therapy.7. Marketed formulation of Niosome:-
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8. Recent development / future advancement:-8.1. Anti glaucomatic niosomal system: recent trend in ocular drug delivery & research-
Glaucoma is adisease and characterized by an intraocular pressure higher than the eye can tolerate. The chronic glaucoma with open angle creates a major problem of public health and it is the second leading cause of blindness in the world. Because of the constraints of physiological factors such as lachrymal drainage, lower culdesacvolume, reflex tearing, drug spillage, and lower corneal permeability onto the cheek; the ocular bioavailability of conventional ophthalmic preparations is very poor. Conventional preparations require frequent instillation, and long term use of such preparations can cause ocular surface disorders. In recent years, significant efforts have been directed towards the development of new carrier systems for ocular drug delivery. Among these, non]ionic surfactant vesicles i.e. niosomes could be a potential one for the effective treatment of glaucoma patients and have gained popularity in ocular drug delivery research. This article reviews the constraints of conventional ocular therapy, complications of glaucoma therapy, and newer advances in the field of anti-glucomatic niosomal formulation.
The commonly used antiglaucomadrugs and their mechanism of action-
Drug/ MOA/ FDA approved medication-
Prostaglandins (lanatoprost, travoprost)/ better outflow of fluids/ Lumigan (Allergan), Travatan (Alcon), Rescula (Novartis), Xalatan (Pfizer)
b.Beta blockers (timolol, betaxolol, levobunolol)/ decreasing fluid production/ Timoptic XE (Merck), Istalol (ISTA), Betoptic S (Alcon).
carbonic anhydrase inhibitors (dorzolamide, acetazolamide, brinzolamide) /decreasing rate of aqueous humor production /Trusopt (Merck), Azopt (Alcon)
d. á2 adrenoceptor agonist (brimonidine, apraclonidine)/ increasing uveoscleral outflow and decreasing aqueous production/ Iopidine (Alcon), Alphagan P (Allergan)
e.Epinephrine decreasing the rate of aqueous humor production and increasing the outflow Propine (Allergan)
8.2.Development of a topical niosomal preparation of acetazolamide: preparation and evaluation-
Orally administered acetazolamide has a limited use in glaucoma due to the systemic side effects associated with its use. No topical formulation of acetazolamide is available, mainly because of it having a limited aqueous solubility and poor corneal permeation. To enhance the bioavailability of acetazolamide by the topical route and to improve the corneal permeability of the drug, niosomes of acetazolamide were prepared (employing span 60 and cholesterol) by different methods. Transmission electron microscopy (TEM) of the selected formulation was carried out to study the morphology. Niosomes were also prepared in the presence of dicetyl phosphate and stearylamine to obtain negatively and positively charged vesicles, respectively. It was found that the reverse-phase evaporation method (REV) gave the maximum drug entrapment efficiency (43.75%) as compared with ether injection (39.62%) and film hydration (31.43%) techniques. Drug entrapment efficiency varied with the charge and the percent entrapment efficiency for the REV method was 43.75, 51.23 and 36.26% for neutral, positively charged and negatively charged niosomes, respectively. Corneal permeability studies, however, showed that the percent permeation and the apparent permeability coefficient for the charged niosomes were less than for the neutral ones. A bioadhesive niosomal formulation of acetazolamide was also prepared and compared with the positively charged formulation, considering that both of them would have a prolonged stay in the cul-de-sac because of their expected interactions with mucin. The formulations were also compared based on their intraocular pressure (IOP)-lowering capacity. The positively charged niosomes (REV2), although showing good corneal permeability and pharmacodynamics, were however found to be inappropriate in terms of the corneal cell toxicity. The bioadhesive coated formulation (REV1bio) compared well with REV2 and also showed a much lesser toxicity. Further, the IOP-lowering effect of the developed formulations was compared with that of a marketed formulation of dorzolamide 2%, a topical carbonic anhydrase inhibitor. The developed niosomal formulations of acetazolamide showed a comparable physiological effect (33% reduction of IOP in REV***1bio and 37% reduction in dorzolamide) with a duration of up to 6 h (the duration being 3 h for dorzolamide). Results of the study indicate that it is possible to develop a safe (as indicated by corneal toxicity studies) and physiologically active topical niosomal formulation of acetazolamide relative in efficiency to the newer local carbonic anhydrase inhibitor, dorzolamide. The developed formulations can form a cost effective treatment plan, which is especially important in the treatment of glaucoma, a chronic ailment affecting middle-aged to old patients.
8.3. Advances in novel parentral drug delivery systems-
The parenteral administration route is the most effective and common form of delivery for active drug substances with poor bioavailability and the drugs with a narrow therapeutic index. Drug delivery technology that can reduce the total number of injection throughout the drug therapy period will be truly advantageous not only in terms of compliance, but also to improve the quality of the therapy. Such reduction in frequency of drug dosing is achieved by the use of specific formulation technologies that guarantee the release of the active drug substance in a slow and predictable manner. The development of new injectable drug delivery system has received considerable attention over the past few years. A number of technological advances have been made in the area of parenteral drug delivery leading to the development of sophisticated systems that allow drug targeting and the sustained or controlled release of parenteral medicines
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