Spectrophotometric Determination of Cilostazol in Tablet Dosage Form

Purpose: To develop simple, rapid and selective spectrophotometric methods for the determination of cilostazol in tablet dosage form. Methods: Cilostazol was dissolved in 50 % methanol and its absorbance was scanned by ultraviolet (UV) spectrophotometry. Both linear regression equation and standard absorptivity were calculated and both methods were validated as per ICH guidelines. Cilostazol was determined in tablet dosage form using these validated methods. Results: The λmax of cilostazol was 258.2 nm in 50 % methanol. Beer-Lambert’s law was obeyed in the concentration range of 0 – 25 μg/ml and standard absorptivity was 420.2 dL.g.cm. The numerical values for all the validation parameters were within acceptable limits. The results of cilostazol tablet determination by linear regression equation and standard absorptivity methods indicate purity of 100.0 102.4 and 98.7 101.1 % with standard deviations of 0.611 and 0.592, respectively. Comparing the methods at 99 % confidence limit, the F-test value was found to be 1.065. Conclusion: These validated methods may be useful for routine analysis of cilostazol as bulk drugs, in dosage forms as well as in dissolution studies in the pharmaceutical industry.


INTRODUCTION
Cilostazol, whose chemical name is 6-[4-(1cyclohexyl-1H-tetrazol-5-y1) butoxy]-3, 4dihydro-2 (1H) -quinolinone (see Fig 1), is a quinolinone derivative that inhibits cellular phosphodiesterase III, and is used for the inhibition of platelet aggregation and as a vasodilator [1][2][3][4].The literature reveals that chromatographic methods are employed in the determination of cilostazol in tablet dosage form and human plasma and, to the best of our knowledge, no spectrophotometric method has yet been reported for this compound in tablet form [5][6].However, the degradation profile and RP-HPLC analysis of cilostazol in tablet dosage form has been reported [7].

EXPERIMENTAL Instruments, reagents and chemicals
Ultraviolet spectrophotometer (1700 series, Shimadzu) and UV-VIS double beam spectrophotometer 2201 (Systronics) with 1 cm matched quartz cells were used for the measurement of absorbance.Shimadzu-Ax-200 electronic balance was used for weighing the samples, and class "A" volumetric glassware were used.
Working standard (WS) of cilostazol was a gift from IPCA Laboratories Ltd, Ratlam, MP, India, while Pletoz ® 50 tablets (cilostazol tablets 50 mg) manufactured by Hetero Drugs Ltd, Hyderabad were procured from a local pharmacy.Methanol AR grade (Merck, India) and distilled water were used for analytical work.

Linear regression equation method
Stock A (500 µg/ml cilostazol) was prepared from accurately weighed 50 mg cilostazol WS in 50 % aqueous methanol.It was diluted with the solvent to produce Stock B (50 µg/ml); aliquots of Stock B were further diluted to give concentrations of 5, 10, 15, 20 and 25 µg/ml of cilostazol, respectively.These dilutions were scanned from 300 to 200 nm against 50 % aqueous methanol as blank, and their absorbances observed at 258.2 nm.

Standard absorptivity method
Five dilutions of cilostazol were prepared in triplicate and their absorbances were observed at 258.2 nm.From the above observations, the standard absorptivity, A (1%, 1cm), and molar extinction coefficient were calculated.

Validation of methods
As per ICH guidelines [8-9], six dilutions in triplicate were used to validate both methods for linearity, accuracy (by recovery studiesstandard addition to pre-analysed samples), repeatability (within day), intermediate precision (days, analyst and instrument variation) and robustness (methanol variation: 45, 50 and 55 %), and statistical parameters were calculated for them.

Quantitative determination
Twenty cilostazol tablets were weighed and finely powdered; a quantity equivalent to 50 mg of cilostazol was dissolved in 100 ml of 50 % aqueous methanol and filtered through Whatman filter paper no.41 to give Stock P. Stock P was diluted to obtain Stock Q (50 µg/ml).Aliquots of Stock Q were diluted to obtain sample concentrations in the range of linearity.The absorbance values of these sample solutions were observed in a multipoint calibration curve of quantitative mode at the selected wavelength (258.2 nm) to obtain test sample concentration.

RESULTS
The linear regression equation obtained from analyzing standard solutions of cilostazol is shown in Eq 1.
It showed an r 2 = 0.9999 where ABC = absorbance, C = concentration (µg/ml), and r 2 = correlation coefficient.The standard absorptivity, A (1%, 1cm), and molar extinction coefficient (ε) for cilostazol were 420.23 dlg   The validated methods were applied to determine cilostazol in tablet dosage form, and the results were 101.23 % (SD = 0.611) by LRE method, and 99.96 % (SD = 0.592) by SA method.Mean of standard deviation (SEx) and standard error of standard deviation (SEσ) were far less than acceptable limits (see Table 3).Value of coefficient of variance for robustness was within acceptable limits.

DISCUSSION
The linear regression equation and standard absorptivity methods have been validated as per ICH guidelines, and results of validation parameters were within acceptable limits.Cilostazol was estimated by LRE and SA methods in tablet dosage form with standard deviation of 0.611 and 0.592, respectively.On comparing the two methods using the Ftest, the calculated value of F (1.065) was substantially less than the theoretical value at 99 % confidence value; therefore, the methods have comparable precision.Impurities or any other interfering substances with λ max close to that of cilostazol (258.2 nm) would adversely affect the accuracy of the developed methods.. Further study on possible interfering substances will need to be carried out.

CONCLUSION
The developed methods for cilostazol are simple, rapid and economical with acceptable accuracy, precision, reproducibility and are robust to slight variations in experimental conditions.Thus, the validated methods may be used for routine analysis of cilostazol as the bulk drug and in tablets and other dosage forms where excipients will not interfere spectrally.

Table 2 :
Results of validation parameters for cilostazol * mean of six dilutions in triplicate; SD = standard deviation; CV = coefficient of variance; SEx = standard error of mean; and SEσ = standard error of standard deviation.

Table 3 :
Results of cilostazol determination in tablets