INTRODUCTION:

Over the past few decades there has been growing interest to develop novel drug delivery systems. These system uses to minimize drug degradation and loss, to prevent harmful side-effects and to increase drug bioavailability and the fraction of the drug accumulated in the required zone, various novel drug delivery systems are currently under development. Among drug carriers one can name non aqueous microemulsion.

Conventional emulsions are heterogeneous system in which one immiscible liquid is dispersed as droplets in another liquid. Such a thermodynamically unstable system is kinetically stabilized by addition of one further components that exhibit emulsify properties.

In emulsion water is an internal phase dispersed in oil are termed as water-in-oil, whereas, emulsion in which the oil is dispersed and water forms the continuous phase are known as oil-in-water emulsions. Emulsion is one of the most convenient and advantageous formulation in which one of the liquid phases is water. However emulsion can be formulated without an aqueous phase to produce anhydrous, non-aqueous or oil in oil microemulsions. Such systems, which can replace conventional emulsions where the presence of water to be avoided.^1-5

Non-aqueous microemulsions have attracted a great deal of attention not only because of their importance in industrial application but also their intrinsic interest. They optimize the performance of a wide spectrum of products and processes. Non-aqueous or anhydrous microemulsions are suitable for poorly aqueous soluble drugs and thermodynamically stable multicomponent fluids composed of polar solvent, oil and mixture of a surfactant. Administration of drugs is one of the convenient and often-advantageous delivery, especially when dealing with children or the elderly for whom pill swallowing can be difficult or even hazardous. Unfortunately many drugs are not soluble in water, while water solution of it may have an unpleasant taste. Some drugs are either unstable in the presence of water or are insoluble in water and therefore cannot be incorporated into aqueous formulations. To overcome these various problems a water free liquid preparation of a number of drugs would be desirable.^6-12

The use of natural and synthetic lipids has generated much academic and commercial interest as a potential formulation strategy for improving the oral bioavailability of poorly water-soluble drugs which administered in traditional solid formulation, these compounds often exhibit low bioavailability as their absorption can be kinetically limited by low rates of dissolution and capacity-limited by poor solubility.

The mechanism behind the solubilization and absorption of poorly soluble compounds is not completely understood. In the fasted state, poorly soluble drugs are believed to be solubilized in a mixed bile salt micelle prior to absorption. When entering the unstirred water layer lining the small intestinal epithelium, the mixed micelle disintegrates and the drug diffuses to the small intestinal epithelium and is absorbed. When introducing the fed state, the processes in the gastrointestinal tract become even more complex. Dietary lipids are hydrolyzed by pancreatic lipase into surface active lipid digestion products that will participate in the formation of complex colloid phased, including vesicles, micelles and liquid laminar phases. In general co-administration with food, increase the bioavailability of poorly soluble drugs, partly due to the solubilizing effects of lipid hydrolysis products, but also due to an increased retention time in the stomach. The anhydrous microemulsion useful for drug delivery which largely overcomes the problem mentioned above with water unstable drug.^13-16

A very palatable formulation can be made in which only one teaspoon contains a normal child’s dose. The emulsion contains a drug dissolved in a suitable non-aqueous internal phase solvent. By drug is meant any therapeutic agents such as vitamins, enzymes, drugs, etc. typical drugs which are suitable include aspirin, indomethacin, ibuprofen, salicylic acid, phenylbutazone, diclofenace, piroxicam, cimetidine, fat soluble vitamins, steroids, vitamins A, D and E and large no. of water unstable compounds such as aspartame. The non aqueous, internal phase of the emulsion is a polar, pharmaceutically acceptable oxygen containing liquid such as polyhydric solvents, polyglycols, etc. The continuous phase of the emulsion is fatty acids (medium chain triglycerides or ethyl palmitate.) etc. The emulsion also contains emulsifier that may be solid, liquid or combination ratio of individual with another emulsifier or may be a novel gum.

Two basics strategies could be considered when searching for stable non-aqueous emulsions. One is to design surfactants having two incompatible blocks, each of which is selectively soluble in either of the immiscible liquids. The other approach is to search for a suitable oil immiscible polar liquid that can substantially replace water using emulsifiers.^17-21

A liquid capable of replacing water in an emulsion should have an appreciable polarity to make it immiscible with oils and to make it a good solvent for the solvophilic part of the surfactant molecules. Hydrogen bonding in the polar liquid is expected to play a role in solvating both ionic and non ionic surfactants and in the formation of a hydrogen bonded network in the liquid itself.

There are many advantages of anhydrous microemulsions like avoidance of phase separations, increased drug loading capacity, carrying capacity of both lipophilic and hydrophilic drugs, Protection against intestinal proteases, increases the bioavailability of the drug, anhydrous microemulsions delivery system can improved the efficacy of drug allowing total dose to be reduced and thus minimizing side effect. Because of thermodynamic stability anhydrous microemulsion are easy to prepare and require no significant energy contribution during preparation. Developed non aqueous system provide better sustained release action with higher bioavailability.^22-27

MATERIALS AND METHODS:

The present work was aimed at formulating non aqueous microemulsion of castor oil and PEG 400, exploring also the possibility of using such systems as non aqueous vehicles for controlled drug release.

Span 40 and Tween 40 were used as the major component, gifted from Ashoj soft gelatin Pvt. Ltd., Vadodra, India. PEG 400 obtained as a gift sample from Loba Chemie. Castor Oil was purchased from Loba Chemie Pvt. Ltd., India. Pure drug cimetidine was purchased form loba chemie.

Methods:

Surfactants (Span 40 and Tween 40) were heated in PEG 400 for 60 minutes in a 55^oC water bath. The mixture was homogenized and the oil phase added slowly. Processing was continued with addition of cimetidine and the microemulsion was progressively cooled to room temperature. The microemulsions were visually observed over time to assess short term stability. Optical microscopy done for size distribution of non aqueous microemulsion. Table 1 shows experimental composition of non aqueous microemulsions.

Table: 1 Composition of Non aqueous microemulsion

S. No.	Ingredients	Quantity
1	Castor oil to PEG weight ratio	50:50
2	Cimetidine	100 mg
4	Span 40 and Tween 40	25:75
5	Homogenization Speed	5000 rpm
6	Homogenization Time	30 min.

Transmission electron microscopy:

Transmission electron microscopy has performed for size and shape by using MORGAGNI 268 D, FEI Neetherland. Experiment has done by fixation of the sample to prevent disruption than apply for dehydration and infiltration with embedding material. Kept it 10 minute for stable state than sectioning and collection of sample followed by staining of the sectioned material for observation.

Diffusion cell technique for in vitro drug release study:

The donor and receiver chambers in the diffusion cell technique usually have equal volume capacities and are separated by a membrane of variable pore size. Formulated non aqueous microemulsion are placed in the donor chamber and the continuous phase is placed in the receiver chamber. The rate of drug appearance in the receiver chamber is analyzed by sampling. However, the membrane surface area available for drug diffusion is small compared with the emulsion surface area, which can lead to violation of sink conditions in the donor chamber and restriction of free drug transport to the receiver chamber. Additionally, sink conditions may be violated due to limited continuous phase in the donor chamber. These diffusion cell systems were found to be most appropriate for characterizing penetration of drugs from formulations.

20 ml of cimetidine loaded non aqueous microemulsion were placed in diffusion cell. This diffusion cell was suspended in 60 ml of phosphate buffer (pH7.4) at 37±1⁰C. The samples (2ml) were withdrawn at definite intervals and absorbance was noted at 216nm by using visible-spectrophotometer. Phosphate buffer pH 7.4 was taken as blank. The volume of buffer was replaced with phosphate buffer after each withdrawn. Release profile of non aqueous microemulsion was obtained.

Entrapment efficiency of cimetidine loaded non aqueous microemulsions:

Sample was separated by centrifugation method. 1 ml of non aqueous microemulsions was taken and add equal quantity of alcohol then aqueous microemulsions was placed in a clear plastic walled ultracentrifuge tube and centrifuge at 10000 rpm for three hours. After spinning, the tube was carefully removed from the rotor. The absorbance of unentraped drug was measured at 320nm after suitable dilution. The quantity of the entrapped drug in the non aqueous microemulsion was calculated.

RESULT AND DISCUSSION:

The present investigation was undertaken with the objective of formulation and evaluation of cimetidine loaded non aqueous microemulsion where Span 40 and Tween 40 were used as the major component. These were combined with PEG 400 and castor oil. Its visual identification and characterization found stable anhydrous or non aqueous microemulsions.

The microscope eyepiece was fitted with a micrometer by which the size of the particles may be estimated. Calibrate the eyepiece micrometer with a standard stage micrometer. Sample (anhydrous microemulsions) were taken on the plain slide in dilute form and measured the size of particles with the help of eyepiece micrometer. Optical microscopy was used to determine average globule size. It gave proper size distribution of globules. Average globule size of anhydrous microemulsions is 0.257 μm. (Fig. 1)

Fig: 1 vesicles of anhydrous microemulsions

Samples were coated with Cu grid and then stained with 2% phosphotungstic acid (PTA) solution and dried under room temperature. The sample was ready for the TEM investigation. Samples were observed at 28,000 magnifications by applying 80Kv energy and found matrix analysis of formulated anhydrous microemulsions.

TEM (MORGAGNI 268 D, FEI Neetherland) was operated at an acceleration voltage of 80 kV and magnification about 28,000. TEM used to describe physical properties and biological fate of anhydrous microemulsions and their entrapped substances in vivo. TEM was conducted with specified manner. Average vesicle size was found to be 0.1µm to 2.0µm. (Fig. 2 a)

Fig: 2 (a) Vesicles of anhydrous microemulsions (size 0.1µm to 2 µm) entrapping with cimetidine

In vitro release studies were performed to predict the drug release of cimetidine from anhydrous microemulsions. Table and graph shows cimetidine release was sustained for 9 hours and percentage cumulative drug release was 59 %. (Table 2 and fig. 3 a, b)

Table: 2 in vitro drug release study of cimetidine loaded anhydrous microemulsions

S. No.	Time (min.)	Concentration (mg)	% Release conc.	Cumulative % release
1.	5	0.163	0.887	0.887
2.	10	0.480	2.61	3.501
3.	15	0.696	3.793	7.294
4.	30	0.920	5.001	12.30
5.	45	1.338	7.28	19.59
6.	60	1.609	8.765	28.35
7.	75	1.625	8.851	37.20
8.	90	0.619	3.372	40.57
9.	120	0.534	3.376	43.47
10.	240	0.621	3.382	46.85
11.	420	0.623	3.394	50.24
12.	540	1.629	8.87	59.11

Fig: 3 (a) In-Vitro Percentage cumulative Drug release Vs Time graph of cimetidine through semi permeable membrane

Triplicate study (mean±SEM) of in vitro drug release study shows sustained release of cimetidine from anhydrous microemulsions. Graph shows cimetidine release was sustained for 9 hours and percentage cumulative drug release was 43.27 %.

Fig: 3 (b) In-Vitro Percentage cumulative Drug release Vs Time graph of cimetidine (mean±SEM) through semi permeable membrane

Amount of cimetidine in supernatant and sediment gave a total amount of cimetidine in 1 ml dispersion.

% entrapment of drug was calculated by the following formula:

Amount of drug in sediment

% Drug Entrapped (PDE) = ________________= X 100 Total amount of drug

Entrapment efficiency of 22.71% was obtained for cimetidine loaded anhydrous microemulsions.

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Received on 16.08.2011 Modified on 29.08.2011

Research J. Pharm. and Tech. 4(11): Nov. 2011; Page 1757-1760