INTRODUCTION:

Transdermal drug delivery system (TDDS) showed promising result in comparison to oral drug delivery system as it eliminates gastrointestinal interferences and first pass metabolism of the drug but the main drawback of TDDS is it encounters the barrier properties of the Stratum Corneum i.e. only the lipophilic drugs having molecular weight < 500 Da can pass through it ^{1 -3}. To improve the permeation of drugs through the skin various mechanisms have been investigated, including use of chemical or physical enhancers, such as iontophoresis, sonophoresis, etc.

Liposomes, niosomes, transferosomes and ethosomes also have been reported to enhance permeability of drug through the stratum corneum barrier. Permeation enhancers increase the permeability of the skin, so that the drugs can cross through the skin easily⁴. Unlike classic liposomes, that are known mainly to deliver drugs to the outer layers of skin, ethosomes can enhance. permeation through the stratum corneum barrier ^{5, 6}. The use of lipid vesicles in delivery systems for skin treatment has attracted increasing attention in confined recent years^7,8. However, it is generally agreed that classic liposomes are of little or no value as carriers for transdermal drug delivery because theydo not deeply penetrate the skin, but rather remain confined to the upper layer of the stratum corneum⁷. Only specially designed vesicles were shown to be able to allow transdermal delivery^9,10.

Ethosomes permeate through the skin layers more rapidly and possess significantly higher transdermal flux in comparison to conventional liposomes ^11-13. Ethosomes (Fig. 1) are lipid vesicles containing phospholipids, alcohol (ethanol and isopropyl alcohol) in relatively highconcentration and water. Ethosomes are soft vesicles made of phospholipids and ethanol (in higher quantity) and water ^1,11. Ethosomes can entrap drug molecule with various physicochemical characteristics i.e. of hydrophilic, lipophilic, or amphiphilic. The size range of ethosomes may vary from tens of nanometers to microns (μ)^14,15. Ethosomes were developed by Touitou et al.,1997, as additional novel lipid carriers composed of ethanol, phospholipids, and water. They are reported to improve the skin delivery of various drugs. Ethanol is an efficient permeation enhancer that is believed to act by affecting the intercellular region of the stratum corneum. Ethosomes are soft malleable vesicles composed mainly of phospholipids, ethanol (relatively high concentration), and water. These soft vesicles represent novel vesicles carriers for enhanced delivery through the skin. The size of the ethosomes vesicles can be modulated from tens of nanometers to microns. Ethosomes are non-invasive delivery carriers that enable drugs to reach the deep skin layers and/or the systemic circulation. The high concentration of ethanol makes the ethosomes unique, as ethanol is known for its disturbance of skin lipid bilayer organization. Therefore, when integrated into a vesicles membrane; it gives the vesicle the ability to penetrate the stratum corneum. Also, because of their high ethanol concentration, the lipid membrane is packed less tightly than the conventional vesicles, although it has equivalent stability, allowing a more malleable structure and improves the drug distribution ability in the stratum corneum lipids.¹⁶

Fig:1 structure of ethosome

Advantages of ethosomal drug delivery:

1. Delivery of large molecules (peptides, protein molecules) is possible.

2. It contains non-toxic raw material in formulation.

3. Enhanced permeation of drug through skin for transdermal drug delivery.

4. Ethosomal drug delivery system can be applied widely in Pharmaceutical, Veterinary, Cosmetic fields.

5. High patient compliance: The ethosomal drug is administrated in semisolid form (gel or cream) hence producing high patient compliance.

6. Simple method for drug delivery in comparison to Iontophoresis and Phonophoresis and other

complicated methods.

MECHANISM OF DRUG PENETRATION:^17,18

The main advantage of ethosomes over liposomes is the increased permeation of the drug. The mechanism of the drug absorption from ethosomes is not clear. The drug absorption probably occurs in following two phases:

1. Ethanol effect

2. Ethosomes effect

1. Ethanol effect:

Ethanol acts as a penetration enhancer through the skin. The mechanism of its penetration enhancing effect is well known. Ethanol penetrates into intercellular lipids and increases the fluidity of cell membrane lipids and decrease the density of lipid multilayer of cell membrane.

2. Ethosomes effect:

Increased cell membrane lipid fluidity caused by the ethanol of ethosomes results increased skin permeability. So the ethosomes permeates very easily inside the deep skin layers, where it got fused with skin lipids and releases the drugs into deep layer of skin.

ETHOSOMES

ETHANOL CAUSE SKIN DISTRUPTION

INCREASE LIPID FLUDITY

MORE PERMEATION THROUGH SKIN

ETHOSOMES PERMEATES INSIDE

FUSED WITH SKINS

RELEASE DRUG INTO DEEP SKIN LAYERS

Figure 2: Mechanism of Action of Ethosomes

Method of prepartion:^19,20

There are two methods which can be used for the formulation and preparation of ethosomes. Both of the methods are very simple and convenient and do not involve any sophisticated instrument or complicated process. Ethosomes can be formulated by following two methods

Hot method:

In this method disperse phospholipids in water by heating in a water bath at 400 ⁰C until a colloidal solution is obtained. In a separate vessel properly mix ethanol and propylene glycol and heat up to 400c. Add the organic phase into the aqueous phase. Dissolve the drug in water or ethanol depending on its solubility The vesicle size of ethosomal formulation can be decreased to the desire extent using probe sonication or extrusion method.

Cold method:^21,22

This is the most common and widely used method for the ethosomal preparation. Dissolve phospholipids, drug and other lipid materials in ethanol in a covered vessel at room temperature with vigorous stirring. Add propylene glycol or other polyol during stirring. Heat the mixture up to 300 ⁰C in a water bath. Heat the water up to 300c in a separate vessel and add to the mixture and then stir it for 5 min in a covered vessel. The vesicle size of ethosomal formulation can be decreased to desire extend using sonication or extrusion method.²³ Finally, the formulation should be properly stored under refrigeration.

Various methods ofcharacterization of ethosomes:

1. Vesicle shape:

Ethosomes can be easily visualized by using transmission electron microscopy (TEM)and by scanning electron microscopy (SEM) and optical microscopy ²⁴.

2. Optical Microscope Observation:

The ethosomal dispersion was spread on the glass slide using a glass rod. Formation of multilamella vesicles was confirmed by examining the ethosomal suspension under an optical microscope with the magnification power of 100 X. Photographs of vesicles were taken using Olympus camera ²⁵.

2. Vesicle size and zeta potential:

Particle size of the ethosomes can be determined by dynamic light scattering (DLS) and photon correlation spectroscopy (PCS). Zeta potential of the formulation can be measured by Zeta meter ²⁶.

3. 3. Transition temperature:

The transition temperature of the vesicular rlipid systems can be determined by using differential scanning calorimetry (DSC) ²⁷.

4. Drug entrapment:

The entrapment efficiency of ethosomes can be measured by the ultracentrifugation technique.

5. Drug content

Drug content of the ethosomes can be determined using UV spectrophotometer. This can also be quantified by a modified high performance liquid chromatographic method.

6. Surface tension measurement

The surface tension activity of drug in aqueous solution can be measured by the ring method in a Du Nouy ring tensiometer.

7. Stability studies

The stability of vesicles can be determined by assessing the size and structure of the vesicles over time. Mean size is measured by DLS and structure changes are observed by TEM ²⁸.

8. Skin permeation studies:

The ability of the ethosomal preparation to penetrate into the skin layers can be determined by using confocal laser scanning microscopy (CLSM).

Therapeutic Applications:

Ethosomes, the high ethanol containing vesicles are able to penetrate the deeper layers of the skin and hence appear to be vesicles of choice for transdermal drug delivery of hydrophilic and impermeable drugs through the skin. Various drugs have been used with ethosomal carrier ²⁹.

Because of their unique structure, ethosomes are able to encapsulate and deliver through the skin highly lyphophilic molecules such as cannabinoids, testosterone, and minoxidil, as well as cationic drugs such as propranolol, trihexyphenidil, Cyclosporine A, insulin, Salbutamol etc. Ethosomes provides a number of important benefits including improving the drug’s efficacy, enhancing patient compliance and comfort and reducing the total cost of treatment. Enhanced delivery of bioactive molecules through the skin and cellular membranes by means of an ethosomal carrier opens numerous challenges and opportunities for the research and future development of novel improved therapies ^30,31.

CONCLUSION:

Transdermal route is promising alternative to drug de-livery for systemic effect. Ethosomes has initiated a new area in vesicular research for transdermal drug delivery which can provide better skin permeation than liposomes. The main limiting factor of transdermal drug delivery system i.e. epidermal barrier can be overcome by ethosomes to significant extent. Application of ethosomes provides the advantages such as improved permeation through skin and targeting to deeper skin layers for various skin diseases. Ethosomes has initiated a new area in vesicular research for transdermal drug delivery which can provide better skin permeation than liposomes or hydroalcoholic solution. Ethosomes are soft, malleable vesicles and potential carrier for transportation of drugs. Ethosomes have been tested to encapsulate hydrophilic drugs, cationic drugs, proteins and peptides. Further, research in this area will allow better control over drug release in vivo and long term safety data, allowing the therapy more effective.

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