INTRODUCTION:

Atorvastatin calcium (ATN), (βR, dR)-2-(4-fluorophenyl)-β, d-dihydroxy- 5-(1-methyl ethyl)-3-phenyl-4-[(phenyl amino) carbonyl]-1H-pyrrole-1-heptanoic acid, calcium salt, is a synthetic cholesterol–lowering agent¹. Many analytical methods like HPLC^2-13, Capillary electrophoresis¹⁴, spectrophotophotometry^15-21,TLC²² and HPTLC^23-26have been reported for the determination of ATN alone and in combination with other antihypertensive and lipid lowering drugs.

EXPERIMENTAL:

INSTRUMENT:

Elico double beam UV/Visible spectrophotometer (SL-164) with 1cm matched quartz cells were used for all spectral measurements.

REAGENTS:

All the reagents used were of analytical grade and the solutions were freshly prepared. The reagents used in Method A were MBTH (0.3%) and FeCl₃(0.5%) and in Method B were 1, 2- naphthaquinone-4-sulphonate (0.2%) and NaOH (2%).

PROCEDURE:

Standard Stock Solution:

A standard solution containing 1 mg/ml was prepared by dissolving accurately about 50 mg of ATN in 5 ml of methanol and made up to volume with distilled water and this solution was used for method A and B.

Method A:

Into a series of 10 ml volumetric flasks, 0.1- 0.5 ml (1 mg/ml) of working standard solution was pipette, 1 ml of 0.3% MBTH solution and 1 ml of 0.5% FeCl₃ solution were added and shaken well. The absorbance of green colored chromogen was measured at 659 nm against reagent blank. The amount of ATN present in the sample solution was computed from its calibration curve.

Method B:

Into a series of 10 ml volumetric flasks 1 ml of 1, 2 naphthaquinone-4-sulphonate (0.2%) was pippetted and 1ml NaOH (2%) solution and 0.6-1.4 ml (1 mg/ml) of working standard solution were added separately. The absorbance of wine red colored chromogen was measured at 455.8 nm against reagent blank. The amount of ATN present in the sample solution was computed from its calibration curve.

Preparation of Sample Solution:

Twenty tablets of commercial samples of ATN were accurately weighed, powdered. Tablet powder equivalent to 50 mg of ATN was dissolved in 5 ml of methanol and made up to 50 ml with distilled water; the solution was filtered and analyzed as given under the assay procedure for bulk samples.

Table 1: Optical characteristics of the proposed methods

Parameters	Method A	Method B
λ_max(nm)	659	455.8
Beer’s law limits (mg/ml)	0.01-0.05	0.06-0.16
Molar extinction coefficient (L/mol.cm)	0.8216 x 10⁴	0.1747 x 10⁴
Sandell’s sensitivity (µg/cm²/0.001 absorbance Unit)	0.03	0.11
Regression equation*(y) Slope(m) Intercept(c)	15.6	2.89
Regression equation*(y) Slope(m) Intercept(c)	-0.0268	0.0256
Correlation coefficient	0.9989	0.9999
Range of errors((Confidence limits) (i) 0.05 level (ii) 0.01 level	0.3008	0.2349
	0.3954	0.2755

*y=mx+c, where x is the concentration in mg/ml and y is absorbance unit, *Average of 5 determinations for method A, *Average of 6 determinations for method B

Table 2: Assay of Atorvastatin tablets.

Sample (tablet)	Labeled amount mg	Amount obtained mg^*		^**%Recovery by the proposed method
Sample (tablet)	Labeled amount mg	Method A	Method B	Method A	Method B
1	20	19.98	19.99	99.90	99.95
2	20	19.88	19.95	99.40	99.75

*Average of three determinations, ** After spiking the sample

Results were represented in Table 1. None of the excipients usually employed in the formulation of tablets interfered in the analysis of ATN by the proposed methods.

Recovery studies

To ensure the accuracy and reproducibility of the results obtained, recovery experiments were performed by adding known amounts of pure drug to the previously analyzed formulated samples and these samples were analyzed by the proposed method. The percentage recovered was depicted in Table-1.

RESULTS AND DISCUSSION:

The presence of amino group in ATN enabled oxidative coupling with MBTH and FeCl₃to form green colored chromogen in method A exhibiting λ_maxat 659 nm. In method B, ATN formed wine red colored chromogen with 1,2-naphthaquinone-4-sulphonate and NaOH exhibiting λ_maxat 455.8 nm. The Beer’s law was obeyed by these two methods in the concentration ranges of 0.01-0.05 and 0.06-0.16 mg/ml respectively. The optical characteristics such as Beer’s law limits (mg/ml), Molar extinction coefficient (L/mol.cm), Sandell’s sensitivity (µg/cm²/0.001 absorbance Unit), Regression equation(y), Correlation coefficient calculated from six measurements containing 3/4^th of the amount of upper Beer’s law limits and Range of errors (0.05 levels and 0.01 Confidence limits) were calculated for the two methods of Atorvastatin and reported in Table 1.

To evaluate the validity and reproducibility of the methods, known amount of pure drug was added to the previously analyzed pharmaceutical preparation and the mixtures were analyzed by the proposed methods. The percent recoveries are given in Table 2. The present methods involve the formation of highly stable colored species which makes it easier for the determination of atorvastatin from pharmaceutical dosage forms in a routine manner.

CONCLUSION:

The developed method is simple, rapid, selective and inexpensive and exhibits a fair degree of precision and accuracy. The method does not involve any critical reaction conditions. The proposed method can serve as an alternative method for the routine analysis of ATN in pure drug and in pharmaceutical formulations.

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Received on 23.04.2010 Modified on 12.05.2010

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 895-897