INTRODUCTION:

Telmisartan, 4-((2-n-propyl-4-methyl-6-(1-methyl benzimidazol-2-yl)-benzimidazol-1-yl) methyl)-biphenyl-2-carboxylic acid (Fig. 1) is a new highly selective, non-peptide angiotensin II type 1 (AT1)- receptor antagonist. Telmisartan lowers blood pressure through blockade of the renin– angiotensin–aldosterone system (RAAS) and widely used in the treatment of hypertension^1-2. Literature study reveals many analytical methods^3-7 for estimation of telmisartan in biological fluids. So far, only one assay procedure has been reported for the estimation of telmisartan from pharmaceutical dosage form but was not cost effective for routine analysis. The availability of validated⁸ HPLC method with high sensitivity and selectivity will be very useful for the estimation of telmisartan. The aim of study was to develop a simple, precise, accurate and cost effective reversed-phase HPLC method for the estimation of telmisartan in bulk drug samples and in pharmaceutical dosage form.

EXPERIMENTAL:

Materials and Methods:

Telmisartan was obtained as a gift sample from Virdev Intermediates Pvt. Ltd. Surat, India. All Chemicals and reagents used were of analytical grades and obtained from Qualigens Fine Chemicals, Mumbai, India. Commercially available Telmisartan Tablets (Telisartan-20, Dr. Reddy Laboratories) was procured from local market.

Instrument:

Quantitative HPLC was performed on liquid Chromatograph, Shimadzu HPLC system, equipped with LC-10 AT VP pump, and SPD M-10AVP PDA detector with injection volume 20 μl. A Luna 5 µ C₁₈, 250 × 4.6 mm, particle size 5 µm was used. The HPLC system was equipped with Class-M 10 data station software.

HPLC Conditions:

The contents of the mobile phase were 5 mM Phosphate buffer: acetonitrile, pH 7.4 adjusted by 1 M Sodium hydroxide in the ratio of 60:40 v/v. They were filtered before use through a 0.45 µm membrane filter, and pumped from the respective solvent reservoirs to the column at a flow rate of 1.0 ml/min. The run time was set at 10.0 min and the column temperature was ambient. Prior to the injection of the drug solution, the column was equilibrated for at least 30 min with the mobile phase flowing through the system. The eluents were monitored at 295 nm.

Preparation of Standard Stock solution:

A standard stock solution of the drug was prepared by dissolving 10 mg of Telmisartan in 100 ml volumetric flask containing 20 ml of Methanol, sonicated for about 15 min and then made up to the mark with methanol to get a 100 µg/ml standard stock solution.

Preparation of Sample solution:

An accurately weighed quantity of tablet powder equivalent to 20 mg of telmisartan was transferred to 100 ml volumetric flask containing 20 ml methanol, sonicated for 15 min. and volume was adjusted up to the mark with same solvent to obtain a stock solution of 200 µg/ml. The resulting solution was filtered using 0.45 µm membrane filter and the solution was further diluted to get final concentration 5 µg/ml of telmisartan.

Figure 1: Structure of Telmisartan

Table I: Linear Regression data for Calibration curve

Drug	Telmisartan
Linearity range (μg/ml)	2-14
Slope	61480
Intercept	-10188
Coefficient correlation	0.9979
% RSD	1.03

Table II: Results of HPLC assay and Recovery studies

Brand Name	Label Claim (mg)	% Found ± SD (n=6)	% Recovered ± SD (n=9)
Telisartan-20	20	100.28 ± 0.93	100.31 ± 0.28
Telisartan-20	20	%RSD = 0.93	%RSD = 0.28

Table III: Validation Summary

Validation Parameter	Results
System suitability
Theoretical Plates (N)	3345
Kapacity Factor (K¹)	1.44
Tailing Factor (As)	1.62
Retention Time (t_R)(min)	7.02
LOD (μg/ml)	0.10
LOQ (μg/ml)	0.29

Linearity:

Aliquots of standard Telmisartan stock solution were taken in different 10 ml volumetric flasks and diluted up to the mark with the diluent such that the final concentrations of Telmisartan are in the range of 2-14 µg/ml. Each of these drug solutions (20 µL) was injected into the column, and the peak area and retention time were recorded. Evaluation was performed with PDA detector at 295 nm and a Calibration graph was obtained by plotting peak area versus concentration of Telmisartan (Fig 2). The plot of peak area of each sample against respective concentration of Telmisartan was found to be linear in the range of 2-14 μg/ml with correlation coefficient of 0.9979. Linear regression least square fit data obtained from the measurements are given in Table I. The respective linear regression equation being Y=61480X-10188. The regression characteristics, such as slope, intercept, and %RSD were calculated for this method and given in Table I.

Assay:

20 μl of sample solution was injected into the injector of liquid chromatograph. The retention time was found to be 7.02 min. All measurements were repeated three times for same concentration and from peak area, the amounts of drug was calculated. The results are shown in Table II.

Recovery Studies:

Accuracy was determined by recovery studies of Telmisartan, known amount of standard was added to the preanalysed sample and subjected to the proposed HPLC analysis. Results of recovery study are shown in Table II. The study was done at three different concentration levels.

Figure 2: Calibration curve of Telmisartan by RP-HPLC

RESULTS AND DISCUSSION:

The system suitability tests were carried out on freshly prepared standard stock solution of Telmisartan. Parameters that were studied to evaluate the suitability of the system are given in Table III.

Limit of Detection (LOD) and Limit of Quantification (LOQ):

The limit of detection (LOD) and limit of quantification (LOQ) for Telmisartan were found to be 0.10 and 0.29 μg/ml respectively. The signal to noise ratio is 3 for LOD and 10 for LOQ.

From the typical chromatogram of Telmisartan as shown in fig 3, it was found that the retention time was 7.02 min. A mixture of 5 mM Phosphate buffer and acetonitrile, pH 7.4 in the ratio of 60:40 v/v was found to be most suitable to obtain a peak well defined and free from tailing. In the present developed HPLC method, the standard and sample preparation required less time and no tedious extraction were involved. A good linear relationship (r=0.9979) was observed between the concentration range of 2-14 μg/ml. Low values of standard deviation are indicative of the high precision of the method. The assay of Telmisartan tablets was found to be 100.28 ± 0.93 %. From the recovery studies it was found that about 100.31 ± 0.28 % of drug was recovered which indicates high accuracy of the method. The absence of additional peaks in the chromatogram indicates non-interference of the common excipients used in the tablets. This demonstrates that the developed HPLC method is simple, linear, accurate, sensitive and reproducible.

Figure 3: Typical Chromatogram of Telmisatan by RP-HPLC

REFERENCES:

1. Maryadele J.O. Neil. Eds, In, The Merck Index, 14^th edition, Published by Merck and Co, White House Station, NJ, USA, 2006,1569.

2. Sean C Sweetman. Eds, In, Martindale: The complete drug reference, 35^th edition, Published by Pharmaceutical press, 2006, 1266.

3. N. Torrealday, L. Gonza´lez, R.M. Alonso, R.M. Jime´nez, E. Ortiz, Lastra, J. of Pharm. and Biomed. Anal., 2003, 32, 847-857.

4. Pengfei Li, Yingwu Wang, Yan Wang, Yunbiao Tang, J. Paul Fawcett, Yimin Cui, Jingkai Gu, J. of Chromatogr. B, 2005, 828, 126-129.

5. S.B. Wankhede, M.R. Tajne, K.R. Gupta, S.G. Wadodkar, Indian J. Pharm. Sci., 2007, 69(2) 298-300.

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8. ICH, Q2 (R1) validation of analytical procedures, text and methodology, International Conference on Harmonization, November 2005.

Received on 07.08.2009 Modified on 03.10.2009

Asian J. Research Chem. 2(4):Oct.-Dec. 2009 page 506-508

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