AN UPDATED REVIEW ON TRANSFERSOMES: A NOVEL VESICULAR SYSTEM FOR TRANSDERMAL DRUG DELIVERY

Transdermal route is an interesting option in this respect because a transdermal route is convenient and safe, avoid first pass metabolism, predictable and extended duration of activity, minimizing undesirable side effects, utility of short half-life drugs, improving physiological and pharmacological responses, avoiding the fluctuation in drug levels and inter and intra-patient variations. However it has got its own limitations its inability to transport large molecules, inability to overcome the barrier properties of stratum corneum and many more. Formulating the drug in a transfersome is one such approach to solve these problems. Transfersome, is an ultradeformable vesicle, elastic in nature which can squeeze itself through a pore which is many times smaller than its size owing to its elasticity.


INTRODUCTION
Since the last few years, the vesicular systems have been promoted as a mean of sustained or controlled release of drugs. The word "transfersome" and the underlying concept were introduced in 1991 by Gregor Cevc. The name "Transfero" is derived from the latin word meaning to carry across and the Greek word "soma" for a body 1 . A transfersome is a highly adaptable and stress-responsive, complex aggregate. Its preferred form is an ultra deformable vesicle possessing an aqueous core surrounded by the complex lipid bilayer. Vesicles are water-filled colloidal particles. The walls of these capsules consist of amphiphilic molecules (lipids and surfactants) in a bilayer conformation 2 . These vesicles serve as a depot for the sustained release of active compounds in the case of topical formulations, as well as rate-limiting membrane barrier for the modulation of systemic absorption in the case of transdermal formulations 3 . Transfersomes consist of a phospholipids component along with a surfactant mixture. The ratio of individual surfactants and total amount of surfactants control the flexibility of the vesicle.
The uniqueness of this type of drug carrier system lies in the fact that it can accommodate hydrophilic, lipophilic as well as amphiphilic to drugs 4 . Transfersomes are applied in a non-occluded method to the skin and have been shown permeate through the stratum corneum lipid lamellar regions as a result of the hydration or osmotic force in the skin. Transfersomes can deform and pass through narrow constriction (from 5 to 10 times less than their own diameter) without measurable loss. Transfersomes can pass through even tiny pores (100mm) nearly as efficiently as water, which is 1500 times smaller.

Mechanism of penetration of transfersomes
After penetration through the outermost skin layers, transfersomes reach the deeper skin layer. From there, they are normally washed out into the blood circulation. If it is applied under suitable conditions, resulting in access to all body tissues 11 .

Figure 2: Penetration pathway of transfersomes
The mechanism for penetration includes generation of "osmotic gradient" due to evaporation of water while applying the transfersomes on the skin surface. The transport of these elastic vesicles is thus independent of concentration. This osmotic gradient is developed due to the skin penetration barrier, prevents water loss through the skin and maintains a water activity difference in the viable part of the epidermis. As the vesicles are elastic, they can squeeze through the pores in stratum corneum (though these pores are less than one-tenth of the diameter of vesicles). Transfersomes by enforcing its own route induce hydration that widen the hydrophobic pores of skin, through the widen pores there is gradual release of drug occurs that binds to targeted organ. Transfersomes act as penetration enhancers that disrupt the intercellular lipids from stratum which ultimately widens the pores of skin and facilitate the molecular interaction and penetration of system across skin 12 . Additives and methods for preparation of transfersome Transferosomes composed of phospholipids like phosphatidyl choline which self assembles into lipid bilayer in aqueous environment and closes to forma vesicle. A bilayer softening component (such as a biocompatible surfactant or an amphiphile drug) is added to increase lipid bi layer flexibility and permeability. This second component is called as edge activator 13 .

Vortexing-sonication method
In this method, mixed lipids (i.e. phospha-tidylcholine, EA and the therapeutic agent) are blended in a phosphate buffer and vortexed to attain a milky suspension. The suspension is sonicated, followed by extrusion through poly-carbonate membranes 14 .

Suspension homogenization process
In this process, transfersomes are prepared by mixing an ethanolic soybean phosphatidylcholine solution with an appropriate amount of edge-active molecule, e.g. sodium cholate. This prepared suspension is subsequently mixed with Triethanolamine-HCl buffer to yield a total lipid concentration. The resulting suspension is sonicated, frozen, and thawed for 2 to 3 times 15 .

Modified handshaking process
In this process, the transfersomes are prepared by modified hand shaking, "lipid film hydration technique". Drug, lecithin (PC) and edge activator were dissolved in ethanol: chloroform (1:1) mixture. Organic solvent was removed by evaporation while hand shaking above lipid transition temperature (43°C). A thin lipid film was formed inside the flask wall with rotation. The thin film was kept overnight for complete evaporation of solvent. The film was then hydrated with phosphate buffer (pH 7.4) with gentle shaking for 15 minute at corresponding temperature 15 .

Aqueous lipid suspension process
In this process, Drug-to-lipid ratio in the vehicles is fixed between 1/4 and 1/9. Depending upon the particular formulation type, the composition is preferred. This would ensure the high flexibility of the vesicle membrane in comparison to the standard phosphatidylcholine vesicles in the fluid phase. Specifically, vesicles with the size ranging from 100-200 nm are prepared by using soyphosphatidylcholine with the standard deviation of size distribution (around 30%). This formulation could be prepared by suspending the lipids in an aqueous phase wherein the drug is dissolved 16 . appropriate buffer by centrifuging at 60 rpm for 1 hour at room temperature. At room temperature, the resulting vesicles are swollen for 2 hours. The multilamellar lipid vesicles obtained which are further sonicated at room temperature 16 .

CHARACTERIZATION AND EVALUATION OF TRANSFERSOMES 1. Determination of vesicle diameter
The vesicle size is one of the key issues during the manufacturing process of transfersomes. It gives important information about the control of the preparation technique and can be utilized for process optimisation. Very small vesicles (smaller than 40 nm) are prone to fusion processes due to the high curvature of their bilayer membrane. It can be determined using photon correlation spectroscopy or dynamic light scattering (DLS) method 17 .

Determination of Vesicle shape and type
Transfersomes vesicles can be visualized by TEM, Phase contrast microscopy, etc 18 .

Determination of vesicle size distribution and zeta potential
Vesicle size, size distribution and zeta potential were determined by Dynamic Light Scattering Method (DLS) using a computerized inspection system by Malvern Zetasizer 18 .

Determination of number of vesicle per cubic mm
This is an important parameter for optimizing the composition and other process variables. Nonsonicated transfersome formulations are diluted five times with 0.9% sodium chloride solution.
Haemocytometer and optical microscope can then be used for further study 18 .

Determination of entrapment efficiency
The entrapment efficiency is expressed as the percentage entrapment of the drug added. Entrapment efficiency can be determined by separating the unentrapped drug. After centrifugation (to separate the unentrapped drug), the vesicle can be ruptured 19 .

Determination of drug content
The drug content can be determined using one of the instrumental analytical methods such as modified high performance liquid chromatography method (HPLC) method using a UV detector, column oven, auto sample, pump, and computerized analysis program depending upon the analytical method of the pharmacopoeial drug 19 .

Turbidity measurement
Turbidity of drug in aqueous solution can be measured using nephelometer 21 .

Surface charge and charge density
Zetasizer is used to determine surface charge and charge density of transferosomes. Surface charge and Charge density of transfersomes can be determined using Zetasizer 21 . 9. Confocal scanning laser microscopy study 22 In this technique lipophilic fluorescence markers are incorporated into the transfersomes and the light emitted by these markers used for following purpose: a) Investigation of the mechanism of penetration of transfersomes across the skin. b) Determination of histological organization of the skin, shapes and architecture of the skin penetration pathways. c) Comparison and differentiation of the mechanism of penetration of transfersomes with liposomes, niosomes and micelles.

In-vitro drug release
In vitro drug release study is performed for determining the permeation rate. For determining in vitro drug release, beaker method is used in which transfersomes suspension is incubated at 32°C using cellophane membrane and the samples are taken at different times and then detected by various analytical techniques (UV, HPLC, HPTLC) and the free drug is separated by minicolumn centrifugation, then the amount of drug release is calculated 23

APPLICATION OF TRANSFERSOMES 1. Delivery of insulin
Insulin is generally administered by subcutaneous route that is inconvenient. Encapsulation of insulin into transfersomes (transfersulin) overcomes the problems of inconvenience, larger size (making it unsuitable for transdermal delivery using conventional method) along with showing 50% response as compared to subcutaneous injection 24 .

Delivery of corticosteroids
Transfersomes improves the site specificity and overall drug safety of corticosteroid delivery into skin by optimizing the epicutaneously administered drug dose. Transfersomes based corticosteroids are biologically active at dose several times lower than the currently used formulation for the treatment of skin diseases 24 .

Delivery of proteins and peptides
Transfersomes have been widely used as a carrier for the transport of proteins and peptides. Proteins and peptides are large biogenic molecules which are very difficult to transport into the body, when given orally they are completely degraded in the GI tract and transdermal delivery suffers because of their large size 25 .

Delivery of anticancer drugs
Anti cancer drugs like methotrexate were tried for transdermal delivery using transfersome technology. The results were favorable. This provided a new approach for treatment especially of skin cancer 25 .

Delivery of anesthetics
Transfersome based formulations of local anestheticslidocaine and tetracaine showed permeation equivalent to subcutaneous injections, with less than 10 min. Maximum resulting pain insensitivity is nearly as strong (80%) as that of a comparable subcutaneous bolus injection, but the effect of transferosomal anesthetics last longer.

Delivery of herbal drugs
Transfersomes can penetrate stratum corneum and supply the nutrients locally to maintain its functions resulting maintenance of skin 25 .

CONCLUSION
Transdermal drug delivery system has several advantages but there is a major limitation, transportion ISSN: 2456-8058 of the larger size molecule. That is why vesicular system like transfersomes are developed to overcome these limitations. This carrier system does not depend upon the concentration gradient and mainly works on the principle of hydrotaxis and elasto-mechanics. Transfersomes are highly deployed in the delivery of hormones, proteins, anticancer drugs, anesthetics and insulin transdermally. Transfersomes hold great prospective in delivery of huge range of drug substances which includes large molecules like peptides, hormones and antibiotics, drugs with poor penetration due to unfavorable physicochemical characters. All above discussed properties of this technology conclude its good future in transdermal drug delivery.