There is a swift progress in the NDDS, so as to overpower the restrictions of conventional drug delivery. Some drugs have an optimum concentration range and in the scope of this optimum range maximum benefit is derived. Some drugs can be can be toxic or produce no therapeutic benefit at all if there concentration is above or below this range. On the other hand, the very slow advancement in the efficiency of the treatment of austere diseases has suggested a growing reqirement for a multidisciplinary approach to the delivery of therapeutics to the targets in the tissues.
From this, new ideas on restraining the pharmacokinetics, pharmacodynamics, non-specific toxicity, immunogenicity, misrecognition, and efficiency of drugs were generated. This new strategy often called the drug delivery systems (DDS).The basis of DDS is the interdisciplinary approaches that involve polymer science, pharmaceutics, bioconjugate chemistry and molecular biology.
To reduce the drug degradation and loss to prevent harmful side-effects and to increase drug bioavailability, to increase the fraction of the drug accumulated in the required zone, many drug delivery and drug targeting systems are currently under development. Among drug carriers one can name soluble polymers, micro particles made of insoluble or biodegradable natural and synthetic polymers, liposomes, niosomes and micelles. The carriers can be caused to be slowly degradable, stimuli-reactive for e.g. it can be made pH- or temperature-sensitive and even targeted for e.g., by conjugating them with specific antibodies against certain characteristic components of the area of interest. Targeting is the ability to guide the drug-loaded system to the area of interest. Two major mechanisms can be made prominent for addressing the desired sites for drug release:
- active targeting.
An example of passive targeting is the preferential accumulation of chemotherapeutic agents in the solid tumours as an outcome of the intensified vascular permeability of tumour tissues in comparison with the healthy tissue. A strategy that could allow active targeting requires the surface fictionalization of drug carriers along with ligands that are selectively acknowledged by receptors on the surface of the cells of interest. Since ligand-receptor interactions can be highly selective and hence this interaction allows a more exact targeting of the area of interest.
For developing successful formulations, controlled drug release and following biodegradation are important and potential release mechanism involves:
- desorption of surface-bound /adsorbed drugs;
- Diffusion through the carrier matrix.
- diffusion (in the case of nanocapsules) through the carrier wall;
- carrier matrix erosion;
- a combined erosion /diffusion process.
The mode of delivery can be the differentiation between a drug’s success and failure, as the choice of a drug is often affected by the way the medicine is administered. Sustained (or continuous) release of a drug includes the involvement of polymers that release the drug at a controlled rate due to diffusion out of polymer or either the by degradation of the polymer over time. Pulsatile release is often the preferred method of drug delivery .As pulsatile release closely copy the way by which the body naturally produces hormones such as insulin, so this release is preferred. It is accomplished by using drug-carrying polymers that react to specific stimuli (e.g., exposure to light, changes in pH or temperature).
1.2 ADVANTAGES OF NOVEL DRUG DELIVERY SYSTEM:
- Reduction in the number and frequency of doses required to maintain the desired therapeutic response.
- Reduce the total amount of drug administered over the period of drug treatment.
- Reduced blood level oscillation characteristic of multiple dosing of conventional dosage forms.
- Reduction in the incidence and severity of both local and systemic side effects in association to the high peak plasma drug concentration.
- Protections from the first pass metabolism, gastro intestinal tract degradation and maximizing availability with minimum dose.
- Targeting the drug molecule towards the tissue or organ leads to the reduction of the toxicity to the normal tissues and improved patient compliance.
- Increased efficiency of the drug and Site specific delivery.
- Reduced toxicity / side effects.
- Shorter hospitalisation and increased convenience.
- Workable treatments for previously incurable diseases.
- Potential for prophylactic application.
- Lower health care costs both short & long term.
- Preferable patient compliance.
COLLOIDAL DRUG CARRIERS
Colloidal drug carrier systems such as micellar solutions, vesicle and liquid crystal dispersions as well as the nanoparticle dispersions consisting of small particles of 10-400 nm diameter are very promising drug delivery systems (fig 1). When these formulations are developed, the goal is to obtain the systems with optimized drug loading and release properties, long shelf-life and reduced toxicity. The incorporated drug takes part in the microstructure of the system, and may even affect it due to the molecular interactions, especially if the drug has amphiphilic and/or mesogenic properties.
Micelles formed by the self-assembly of amphiphilic block copolymers (5-50 nm) in aqueous solutions are of great advantage in the case of drug delivery applications. The drugs can be physically trapped into the core of block copolymer micelles and transported at concentrations that can go farther their intrinsic water- solubility.
Liposomes are the form of vesicles that are contained of many, few or just one phospholipids bilayers. Encapsulation of polar drug molecules is possible by the polar character of the liposomal core. Amphiphilic and lipophilic molecules are solubilised in the scope of the phospholipids bilayer in accordance to their affinity towards the phospholipids. Participation of nonionic surfactants in place of phospholipids in the bilayer formation leads to the formation of noisomes.
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TARGETED DRUG DELIVERY SYSTEM (TDDS)
Drug targeting is a phenomenon in which the distribution of drug in the body is in such a manner that the major part of the drug interacts solely with the target tissue at a cellular or sub cellular level. The objective of the drug targeting is to aim at a desired pharmacological response at a selected site without objectionable interactions at other sites. This is especially important in the cancer chemotherapy and enzyme replacement treatment. Drug targeting is the delivery of drugs to the receptors or any other specificied part of the body to which one desires to deliver the drug.
The targeted or site specific delivery of drugs is in fact a very attractive goal as it provides one of the most possible ways to develop the therapeutic index of the drugs.
Earlier work done between late 1960s and the mid 1980s emphasized the requirement for drug carrier systems chiefly to change the pharmacokinetics of the already proven drugs whose efficiency might be improved by altering the rates for metabolism in liver. These approaches generally were not concentrated to achieve site specific or targeted delivery such as getting a cytotoxic drug to cancerous tissue while reserving other normal, though equally sensitive tissue. A number of technological advancements have since been made in the area of parenteral drug delivery leading to the progress of the sophisticated systems. This system allows drug targeting and the well maintained or controlled release of parenteral medicines.
At present drug targeting is accomplished by one or two approaches:
- The first approach involves chemical modification of the parent compound to a derivative which is activated only at the target site.
- The second approach uses carriers such as liposomes, niosomes, microspheres, nano particles, antibodies, cellular carriers (erythrocytes and lymphocytes) and macromolecules to direct drug to its site of action.
Recent promotion have led to the progress of many novel drug delivery system that could cause fundamental change in the method of medication and make available a number of therapeutic benefits.
The goal of any drug delivery system is to supply a therapeutic amount of drug to the proper site in the body to achieve without delay, and then maintain the required drug constant. The ideal drug delivery system delivers drug at a rate commanded by the requirement of the body over the period of treatment and it channels the active entity solely to the area of action.
Paul Ehrlich in 1906 began the era of development for targeted delivery when he pictured in mind a drug delivery mechanism that would target drugs directly to diseased cells. Numbers of carriers were used to carry drug at the target organ / tissue which include immunoglobulin, serum proteins, synthetic polymers, lipid vesicles (liposomes), microspheres, erythrocytes, reverse micelles, niosomes, pharmacosomes etc. Among the various carriers, few drug carriers reached the stage of clinical trials where phospholipids vesicle exhibit strong potential for effective drug delivery at the site of action. These carriers (liposomes) are biologically inert in nature, lacking in any antigenic, pyrogenic or allergic reactions .The components of carriers can be used as the component of biological membrane. Drugs incorporated in liposomes are not activated under physiological conditions and do not cause unfavourable side effects as well.
There are various techniques by which drug can be targeted include (Brahmankar et al., 2001)
- Resealed erythrocytes.
- Monoclonal antibodies.
1.4.1. MERITS OF TDDS
- Targeting of the drug molecule towards the tissue or organ reduces the toxicity to the normal tissues.
- Increased bioavailability.
- Improved treatment of chronic illness where symptoms break through occurs when the plasma level of the drug falls below the MEC.
- The drug is protected from first pass metabolism and GI degradation.
- Improved patient compliance can be achieved due to decrease in amount and frequency of doses administered.
- Biocompatibility can be well achieved.
- Maintenance of therapeutic action of the drug overnight.
- Systemic and local side effects are successfully reduced due to the reduction in the total amount of the drug
1.4.2. LIMITATIONS OF TDDS
- TDDS such as liposomes, resealed erythrocytes and platelets suffer serious stability problems.
- Though monoclonal antibodies show very high degree of site specificity the selection and isolation procedures are too tough.
- If the particle size of TDDS is high then they may be rapidly cleared by RES.
- Magnetically controlled TDDS shows high specificity to superficially located organs and tissues but cannot be targeted to deep seated organs.
- Monoclonal antibodies may sometimes can cause unwanted antigen – antibody reaction which leads to serious consequences.
- Microspheres of particle size more than 50µg can lead to problem of thromboembolism in general circulation.
- Once drug is administered it cannot be removed if an undesirable action is precipitated or if the drug is no longer needed.
- Most of such systems are administered by subcutaneous or intraperitoneal route.
- The vehicles polymer employed should be sterile, hydrogen free, non irritating, biocompatible and biodegradable into non toxic compounds within an appropriate time preferably close to duration of action.
- Drugs having biological half life of 1hr or less are difficult to formulate as controlled release formulation. The high rates of elimination of such drugs from the body need an extremely large maintenance dose which provides 8 – 12 hrs of continuous therapy.
Niosomes or non-ionic surfactant vesicles are microscopic lamellar structures formed on admixture of non-ionic surfactant of the alkyl, dialkyl polyglycerol ether class and cholesterol with the subsequent hydration in the aqueous media (3).
Niosomes are formed from self-assembly of the non-ionic amphiphiles in the aqueous media out coming in the closed bilayer structures (Fig. 1).
The assembly of the amphiphiles into closed bilayers is rarely spontaneous. It also involves some input of energy such as physical agitation or heat. An assembly in which the hydrophobic parts of the molecule are shielded from the aqueous solvent is the result. As a result the hydrophilic head groups enjoy the maximum contact with same. These structures are analogous to liposomes. These structures are able to encapsulate the aqueous solutes. These structures serve as the drug carriers.
The major advantages of Niosomes as drug carriers includes:
- The vesicle suspension is water-based and it offers high patient compliance in comparison with the oily dosage forms.
- Niosomes possess an infrastructure consisting of hydrophilic, amphiphilic and lipophilic moieties together. As a result of this infrastructure it can accommodate drug molecules with a wide range of solubilities.
- The characteristics of vesicle formulation are variable and controllable. Changing vesicle composition, size, lamellarity, tapped volume, surface charge and concentration can control the vesicle characteristics.
- The vesicles may act as a depot and hence releasing the drug in a controlled manner.
- They are osmotically active and stable. As a result they increase the stability of entrapped drug.
- As surfactant requires no special conditions, they are easy for handling and storage.
- They improve the oral bioavailability of poorly absorbed drugs and hence improve skin penetration of drugs.
- They can be made to reach the area of action by the means of oral, parenteral as well as topical routes.
- The surfactants are biodegradable, biocompatible as well as non-immunogenic.
- They improve therapeutic performance of the drug molecules by delayed clearance from the circulation. Hence protecting the drug from biological environment and restricting effects to target cells.
- Niosomal dispersion in an aqueous phase can be emulsified in a non-aqueous phase. This emulsification is done to regulate the delivery rate of drug and administer normal vesicle in external non-aqueous phase.
METHOD OF PREPARATION OF NIOSOMES
Based on the method used for the production and composition of bilayer, niosomes show a wide variation in their properties. Basis of the preparation of noisome is liposome technology. The basic process of preparation is the same i.e. hydration by aqueous phase of the lipid phase which may be either a pure surfactant or a mixture of surfactant with cholesterol. The bioactive material is dissolved in the aqueous phase/organic phase. This is the bioactive material which is entrapped. The different methods used for preparation of niosome are listed as follows:
- Ether Injection method
- Lipid Film Formation (Hand Shaking Method)
- Sonication Method
- Micro fluidization
- Reverse phase evaporation
- Trans membrane pH gradient (inside acidic) Drug Uptake Process (remote Loading)
- Multiple membrane extrusion method
- The “Bubble” Method.
- Formation of niosomes from proniosomes
Method of preparation
1. Ether injection method
In this method, the niosomes are slowly introduced in 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. The ether vaporizes and its vaporization leads to formation of single layered vesicles. The diameter of the vesicle range from 50 to 1000 nm. This range of the diameter depends upon the conditions being used during this method.
2. Hand shaking method (Thin film hydration technique)
The mixture of vesicles forms ingredients like surfactant and cholesterol. This mixture is a 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. On removal of the organic solvent a thin layer of solid mixture is left deposited on the wall of the flask. Rehydration of the dried surfactant film is done with gentle agitation with aqueous phase at 0-60°C.This process forms typical multilamellar niosomes.
Raja Naresh et al /i>(15) prepared thermosensitive niosomes by evaporation of organic solvent at 60°C and a thin film of lipid was left on the wall of rotary flash evaporator resulting from the process of evaporation. The aqueous phase which contains drug was added slowly with alternate starting and stopping of the shaking of flask at room temperature. This procedure is followed by sonication.
The vesicles are produced typically by the method of sonication of solution as described by Cable. In this method an aliquot of the solution of drug 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 .This sonication is done using a sonicator with a titanium probe to yield niosomes.
4. Micro fluidization
Micro fluidization is a latest technique which is used to prepare unilamellar vesicles of definite sized distribution. Basis of this method is submerged jet principle. According to this principle the two fluidized streams interact at ultra high velocities, in micro channels which are very precisely defined and this interaction takes place within the interaction chamber. The arrangement of the impingement of the thin liquid sheet along a common front is in such a way that the energy supplied to the system does not cross the area of niosomes formation and is within it. As a result the niosomes formed have greater uniformity, small size and better reproducibility.
5. Multiple membrane extrusion method
The surfactant, cholesterol and dicetyl phosphate are mixed in chloroform and made into a thin film by the evaporating this mixture. Aqueous drug polycarbonate membranes,ï€ solution is used to hydrate the film and the extrusion product i.e. resultant suspension is placed in the series of up to 8 passages. This is a good method for controlling the size of niosome.
6. Reverse Phase Evaporation Technique (REV)
Cholesterol and surfactant in the ratio 1:1 are dissolved in a mixture of ether and chloroform. An aqueous phase containing drug is added to this. The resulting two phases are sonicated at 4-5°C. After the addition of a small amount of phosphate buffered saline (PBS), the formed clear gel is sonicated further. The organic phase is removed at 40°C under low pressure. The resulting viscous niosome suspension is diluted with PBS. And then it is heated on a water bath at 60°C for 10 min so as to yield niosomes.
7. Trans membrane pH gradient (inside acidic) Drug Uptake Process (remote Loading)
Surfactant and cholesterol are dissolved in chloroform. The solvent then undergoes evaporation under reduced pressure. This results to 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 .After freezing it is thawed 3 times and later sonicated. To this niosomal suspension, aqueous solution containing 10 mg/ml of drug is added. Then this suspension is 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 result to give niosomes.
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8. The “Bubble” Method
It is novel technique for the one step preparation of liposomes and niosomes. In this method organic solvents are not used. 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 is mixed for 15 seconds with high shear homogenizer. Then immediately afterwards “bubbled” at 70°C using nitrogen gas.
9. Formation of niosomes from proniosomes
Another method of producing niosomes is to coat a water-soluble carrier such as sorbitol with surfactant. The product of the coating process is a dry formulation. In this formulation each water-soluble particle is covered with a thin film of dry surfactant (fig c).This preparation is termed as “Proniosomes”. The niosomes are recognized by the adding of aqueous phase at T > Tm .It is followed by brief agitation.
Tm = mean phase transition temperature.
CHARACTERIZATION OF NIOSOMES –
a) Entrapment efficiency
After preparing the niosomal dispersion, unentrapped drug is separated by either the method of dialysis  entrifugation [17-18] or gel filtration  as described above and the drug remained entrapped in niosomes is determined by the complete vesicle disruption using 50% n-propanol or 0.1% Triton X-100.The resultant solution is analysed by the appropriate assay method for the drug. Where entrapment efficiency (EF) = (Amount entrapped total amount) x 100
b) Vesicle diameter
Niosomes, similar to liposomes, assume spherical shape. Such niosomes’s diameter can be determined using either light microscopy or photon correlation microscopy or 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. This result might be attributed to fusion of vesicles during the cycle.
c) In-vitro release
A method of in-vitro release rate study includes the use of dialysis tubing. A dialysis sac is washed. This sac is soaked in distilled water. The vesicle suspension is pipetted into a bag. The bag is 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  .
d) Osmotic Shrinkage
Osmotic shrinkage of vesicles can be determined by monitoring reductions in vesicle diameter. This method is initiated by the addition of hypertonic salt solution to the suspension of niosomes. Niosomes prepared from pure surfactant are osmotically more sensitive in comparison to vesicles containing cholesterol.
e) Physical stability of vesicles at different temperature
Aggregation or fusion of vesicles as a function of temperature was determined as the changes in vesicle diameter by laser light scattering method. The vesicles were stored in glass vials at room temperature or kept in refrigerator (4oC) for 3 months. The changes in morphology of multilamellar vesicles (MLVs) and also the constituent separation were assessed by an optical microscope. The retention of entrapped drug was measured 72 hours after preparation. It is again measured after 1, 2 or 3 months in same formulations
f) Turbidity Measurement
The niosomes were diluted with bidistilled water to give total lipid concentration of 0.312 mM.After rapid mixing by sonication for 5 min; the turbidity was measured as the absorbance with an ultraviolet-visible diode array spectrophotometer.
Niosomal drug delivery is potentially applicable to many of the pharmacological agents for their action against various diseases. Some of their therapeutic applications are discussed below.
Targeting of bioactive agents
To reticulo-endothelial system (RES)
The cells of RES preferentially take up the vesicles. The uptake of niosomes by the cells is by circulating serum factors known as opsonin.This factor which mark them for clearance. Such localized drug accumulation has nevertheless been exploited in the treatment of animal tumors known to metastasize to the liver, spleen and in parasitic infestation of liver.
To organs other than RES
It has been suggested that carrier system can be directed to specific areas in the body by use of antibodies. Immunoglobulins seem to bind quite readily to the lipid surface and hence offering a convenient means for targeting of drug carrier. Many cells possess the intrinsic ability to recognize and bind particular carbohydrate determinants. This can be exploited to direct carriers system to particular cells.
The anthracyclic antibiotic named doxorubicin has broad spectrum anti tumor activity.This antibiotic shows an irreversible cardio toxic effect. This effect is dose dependent. The niosomal delivery of this drug to mice bearing S-180 tumor lead to the growth in their life span. This delivery reduced the rate of proliferation of sarcoma. The entrapment of the niosome increased the half-life of the drug and extends duration of its circulation, hence lead to alteration in its metabolism. The administration of the methotrexate intravenously entrapped in the niosomes to S-180 tumor bearing mice lead to the total regression of tumor ,higher plasma level and slower elimination.
In the treatment of diseases where the infecting organism dwells in the organ of reticulo-endothelial system, niosomes can be used for targeting the drug. A disease in which the parasite enters and causes harm to cells of liver and spleen is Leishmaniasis. The commonly prescribed drugs are antimonials, which have relation with arsenic. The antimonials at high concentration will affect and damage heart, liver and kidney.
4) Delivery of peptide drugs
When oral delivery of 9-desglycinamide, 8-arginine vasopressin is entrapped into the niosomes in an in-vitro intestinal loop model, it was reported that the peptide’s stability increased consequently.
5) Immunological application of niosomes
The nature of the immune response aroused by the antigens has been studied using the niosomes. In terms of the immunological selectivity, low toxic level and stability the niosomes act as potent adjuvant.
6) Niosomes as carriers for Hemoglobin
Niosomes can act as a carrier for hemoglobin. The suspension of niosomes points a visible spectrum super imposable to the free hemoglobin. The vesicles are permeable to oxygen and dissociation curve for hemoglobin can be moderated in the similar manner as to non-encapsulated hemoglobin.
7) Transdermal delivery of drugs by niosomes
The slow penetration of the drug via skin is the major disadvantage of the transdermal route of delivery. A growth in penetration rate has been accomplished by the transdermal delivery of drug included as a part in niosomes. Niosomes on hairless mouse are the various formulations based on the topical delivery of erythromycin. As per the studies and the confocal microscopy, it was found that non-ionic vesicles could be devised to target the pilosebaceous glands.
8) Other Applications
The liver plays the role of depot for methotrexate, after niosomes the role of depot for metotexate is played by the liver cells. The released action of niosomes is applicable to the drugs with low therapeutic index and having low water solubility since these conditions for the drug can be maintained in the circulation through the encapsulation of niosome.
Localized Drug Action
The delivery of drug through niosomes is one of the approaches to accomplish the localized drug action, since the size and low penetrability of the niosomes through the epithelium and connective tissue keeps the drug confined at the area of administration.
The localized drug action results in the improvement of the effectiveness of the potency of the drug and at the corresponding time it decreases its systemic toxic effects e.g. Antimonials encapsulated within niosomes are received up by the mononuclear cells which results in the localization of the drug, increase in potency and therefore reduce in dose as well as toxicity.
PRONIOSOMES – A STABLE PRECURSOR FOR NIOSOMES
Proniosomes offer a variety vesicle drug delivery concept with latent ability for delivery of drugs (Gupta et al., 2007).These dry niosomes is dry free flowing; granular product upon addition of water. The dry niosomes disperses or dissolves to form a suspension of multilamellar niosomes.
Preparation of proniosomes
There are many components present in proniosomes with non -ionic surfactants and cholesterol, among them lecithin is the main ingredient. The required characteristics of the selected carrier that could be used in the preparation of proniosomes were formally speeched by Payne et al. (2008).These include: safety and non-toxicity, free flowability,poor solubility in the loaded mixture solution and good water solubility for easy hydration (Abd-Elbary et al.,2008). In the proniosome preparation, different carriers and non ionic surfactants and membrane stabilizers are used. As shown in table 1. Three different methods were reported for the preparations of proniosomes .They are:
Slurry method: Maltodextrin powder 10 g as carrier is added to a 250-mL round-bottom flask and the whole volume of the surfactant solution (14.5 mL) was mixed directly to the flask to form slurry. If the surfactant solution volume is less, then some extra amount of organic solvent can be mixed to get slurry. The flask was attached to the rotary evaporator and vacuum was applied until the powder appears to be dry and having free flow. The flask was removed from the evaporator and kept under vacuum overnight. Proniosome powder was stored in the sealed containers at 4°C. The time needed to produce proniosomes is not dependent on the ratio of surfactant solution to carrier material and appear to be scalable (Blazek-Welsh and Rhodes, 2001a and b; Solanki et al., 2007 and Perrett et al., 1991).
Coacervation phase separation method: This method is widely adopted to formulate proniosomal gel. In a clean and dry wide mouthed glass vial of 5.0 ml capacity exactly weighed amounts of surfactant, lipid and drug are taken and alcohol (0.5 ml) is added to it. After warming, all the ingredients are mixed well with a glass rod. Then the open end of the glass bottle is covered with a lid to prevent the loss of solvent from it and then it is warmed over water bath at 60-70°C for about 5 min until the surfactant mixture is dissolved completely. Then the aqueous phase (0.1% glycerol solution) is added and warmed on a water bath till a clear solution was formed which then gets converted into proniosomal gel when it is cooled (Vora et al.,1998 and Gupta et al., 2007).
Slow spray-coating method: This method includes the formulation of proniosomes by spraying surfactant in the organic solvent onto sorbitol powder and then evaporating the solvent. It is important to repeat the process until the desired surfactant loading has been achieved because the sorbitol carrier is soluble in the organic solvent. The surfactant coating on the carrier is very thin and hence hydration of this coating will allow multilamellar vesicles to form as the carrier dissolves (Bangham et al., 1965 and Yoshioka et al., 1994). The resulting niosomes are very similar to
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