INTRODUCTION:

There are so many techniques, which are employed for the enhancement of solubility of insoluble drugs. Solubility is an important phenomenon required for the development of a formulation because low solubility can limits development of parental products and also decreases bioavailability of orally administered dosage form. However, various organic solvents like methanol, chloroform, DMF, DMS have been employed for the solubilization of poorly water soluble drugs for spectrophotometric estimation [1]. But these all co-solvents have toxicological liabilities, higher cost, pollution, error in analysis due to volatility and are not acceptable for use in clinical formulations. In such cases hydrotropic solubilization technique is utilized for the improvement of solubility of insoluble drugs.

Hydrotropes (non-micelle-forming) are substances that solubilize hydrophobic compounds in aqueous solutions. They are either liquids or solids, organic or inorganic, capable of solubilizing insoluble compounds. Hydrotropes do not have a critical concentration above which self-aggregation 'suddenly' starts to occur. The term hydrotropy was originally put forward by Carl Neuberg to describe the increase in the solubility of a solute by the addition of fairly high concentrations of alkali metal salts of various organic acids [2].

Hydrotropy is a phenomenon where the presence of a large quantity of one solute enhances the solubility of another solute [3]. Sodium salicylate, sodium benzoate, urea, nicotinamide, sodium citrate and sodium acetate are the most common examples of hydrotropic agents utilized to increase the water solubility of drug [4-17].

Dextromethorphan is the dextrorotatory enantiomer of levomethorphan, which is the methyl ether of levorphanol, both opioid analgesics. It is named according to IUPAC rules as (+)-3-methoxy-17-methyl-9α, 13α, 14α-morphinan. As the pure free base, dextromethorphan occurs as an odorless, white to slightly yellow crystalline powder. It is freely soluble in chloroform and insoluble in water [18]. Dextromethorphan is commonly available as the monohydrated hydrobromide salt, however some newer extended-release formulations contain dextromethorphan bound to an ion exchange resin based on polystyrene sulfonic acid.

The present study aims to develop, simple, accurate, ecofriendly, cost effective, safe, sensitive hydrotropic spectrophotometric methods for the comparative analysis of standard dextromethorphan with tablet formulation of the same.

EXPERIMENTAL:

Chemicals and Instruments:

Reference Dextromethorphan was generous gift from Wockhardt Pvt. Ltd. Baddi (India). Urea used in the study was of analytical grade. Shimadzu UV-visible spectrophotometer (model UV-1700series), having double beam detector configuration with 1 cm matched quarts cells was used in the study.

Preliminary solubility studies/Saturation solubility studies:

Solubility of Dextromethorphan was determined at (28±2)° C. An excess amount of drug was added to 25 ml volumetric flasks containing 15ml of different aqueous systems viz. double distilled water, Urea (1, 2, 4, 6, 8 10 M), Ascorbic Acid (1, 2, 4, 6, 8 M), Citric Acid (1, 2, 4, 6, 8 M), and sodium acetate (1, 2, 4, 6, 8 M) solution. Enhancement of solubility of drug was increased by 1.8 folds in 8 M urea. This enhancement of solubility was due to the hydrotropic solubilization phenomenon. The enhancement ratio in solubility was determined by the following formula:

Optimization -Selection of hydrotrope:

Different available hydrotropic solubilizers including double distilled water, Ascorbic Acid (1, 2, 4, 6, 8 M), Citric Acid (1, 2, 4, 6, 8 M), Urea (1, 2, 4, 6, 8, 10 M) and sodium acetate (1, 2, 4, 6, 8 M) solutions were used for optimization at room temperature.

UV spectral studies:

In order to check any interaction between drug and the hydrotropic agent, UV spectral studies of Dextromethorphan were performed in different concentration of hydrotropic solutions. Possible spectroscopic changes in the structure of Dextromethorphan in the presence of Hydrotropes were subsequently investigated.

Method development:

Preparation of stock solution:

Accurately weighed 50 mg of the Dextromethorphan drug sample was transferred into 50 mL volumetric flask containing 5mL of 8 M urea solution, shaken, sonicate for 12 min and diluted up to 50 mL with double distilled water and filtered through Whatman filter paper no.1. The 5 ml of filtered solution was further diluted to 50 mL with double distilled water to prepare stock solution (100 µg/ml).

The fresh aliquot of 20 g/mL was prepared from stock solution and scanned in the spectrum mode from 200 nm - 400 nm wavelength range on spectrophotometer.

Analytical characteristics of the proposed methods:

By using the proposed methods, the different optical characteristics of hydrotrope Dextromethorphan such as absorption maxima and Beer’s law limit, were calculated. The regression analysis using the method of least squares was made for the slope (m), intercept (c) and correlation coefficient (r²) obtained from different concentrations.

Method validation:

The method was validated in accordance of ICH (2005) [19] for validation of analytical procedures in order to substantiate linearity and range, precision, recovery, robustness, LOD and LOQ for each method.

RESULT AND DISCUSSION:

Optimization - Selection of hydrotrope:

Dextromethorphan being insoluble in water, was selected for the application of hydrotropy phenomenon. After assessing their solubility pattern 8 M urea was selected as working hydrotropic solubilizing agent for analysis. The pH of 8 M urea was 8.56. The solubility enhancement of Dextromethorphan is not entirely due to pH effect, but is largely due to hydrotropy.

UV spectral studies:

Urea does not show any absorbance above 278nm. The other excipients in composition do not show any absorbance in analyzing range of Dextromethorphan. Thus the hydrotropic agent as well as excipient did not interfere in the analysis of Dextromethorphan.

Analytical characteristics of the proposed methods:

The different optical characteristics of hydrotropic Dextromethorphan were calculated for the proposed method and results are mentioned in Table No. 1

Table No. 1 Optical parameters of hydrotrope Dextromethorphan for proposed method:

Parameter	Proposed Method
Wavelength	278nm
Beer’s Law limit	10-120μg/ml
Regression Equation	y=0.0022x
Slope (m)	0.0022
Intercept (c)	0.000
Correlation Coefficient (r²)	0.9993

On scanning, maximum absorbance was observed at 278 nm and hence 278 nm was selected wavelength. Calibration curve was plotted between concentration verses absorbance shows obeying the Beer’s-Lambert’s Law in the range (1-120μg/ml). Drug content was calculated as per the following Beer’s- Lambert’s equation

A= a, b, c

Method validation:

The validation of an analytical method confirms the characteristics of the method to satisfy the requirements of the application. Under the validation study the following parameters were studied and summarized result are shown in Table No. 2.

Table No. 2 Validation Parameters:

S. No.	Validation Parameters	Results
1	Linearity (r²)	0.9993
2	Range	10-120μg/ml
3	Precision (%RSD) Intra Day Inter Day	0.8182 0.9438
4	Recovery Study (Average Mean ecovery)	99.85
5	LOD	3.76 μg/ml
6	LOQ	11.41 μg/ml

Linearity and Range:

A linearity curve was plotted between the concentration of the hydrotrope Dextromethorphan and absorbance. The absorbance was found to be linear over analytical range of 10-120μg/ml with regression coefficient value of 0.9993 against water as blank as shown in Figure No. 2.

Figure No. 1 Linearity Curve

Precision:

Precision was studied to find out intraday and inter-day variation in test methods of Dextromethorphan in the concentration ranges of 10µg/ml to 120µg/ml. for three times on the same day and later day. Precision was determined by analyzing corresponding standard daily for a period of three days. The % RSD in case of intra-day and inter day was found to be 0.8182 and 0.9438 respectively.

Recovery study:

Accuracy was determined by recovery studies of Dextromethorphan, a known amount of Dextromethorphan reference standard was added it to preanalyzed sample and subjected them to the proposed method. Results of the recovery study were shown in Table No. 3.The study was carried out at three different concentration levels.

Table No. 3 Recovery Study of Dextromethorphan

Drug(mg)

Amount Added (mg)

Amount recovered* (mg)

% Recovered

Dextromethorphan (25mg)

10.1

20.1

30.0

35.4±0.05

45.2±0.01

55.1±0.07

100.5

100.1

99.25

Mean

99.95

*Each value is mean deviation of three determinations

LOD and LOQ:

The LOD and LOQ for hydrotrope Dextromethorphan were calculated from the slope (m) of the calibration plots and the standard deviation (SD) of the blank using the following equation:

LOD=3.3 σ/S

LOQ=10 σ/S

Where, σ is Standard Deviation and S is Slope

The LOD for hydrotrope Dextromethorphan was found to be 3.76μg/ml while LOQ was found to be 1.141 μg/ml.

CONCLUSION:

The method described in this paper for the estimation of Dextromethorphan is found to be simple, sensitive, accurate, precise, rapid and economical. The value of standard deviation and % RSD were indicating of the accuracy of the proposed method. Also use of hydrotropic agent further enhance the aqueous solubility of the Drug. Hence the method could be successfully employed for the routine analysis of Dextromethorphan

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Received on 26.07.2013 Accepted on 01.08.2013

Asian J. Pharm. Ana. 3(3): July-Sept. 2013; Page 90-93