Development and Validation of Spectrophotometric Determination of Glimepiride in Pure and Tablet Dosage forms through Ion-Pair Complex Formation using Bromothymol Blue

 

Abdul Aziz Ramadan*, Souad Zeino

Department of Chemistry, Faculty of Science, University of Aleppo, Syria.

 

ABSTRACT:

A simple, direct and accurate spectrophotometric method has been developed for the determination of Glimepiride (GLM) in pure and pharmaceutical formulations by complex formation with bromothymol blue (BTB). The method involves the formation of a yellow ion-pair complex between BTB with glimepiride (C₂₄H₃₄N₄O₅S) at pH<3,8; after reacting GLM with Na₂CO₃ to give C₂₄H₃₃N₄H⁺O₅NaS which is extracted by chloroform. The formed complex [GLM]:[BTB] was measured at ^λmax 412 nm against the reagent blank prepared in the same manner. Variables were studied in order to optimize the reaction conditions. Molar absorptivity (ε) for complex was 18024 L mol^-1 cm^-1. Beer’s law was obeyed in the concentration range of 2.453 - 49.062 μg.mL^-1 in present of 5.0x10^-4 mol.L^-1 of BTB with good correlation coefficient (R²= 0.9995). The relative standard deviation did not exceed 3.8%. The limit of detection (LOD) and the limit of quantification (LOQ) were 0.30 and 0.91 μg.mL^-1, respectively. The proposed method was validated for specificity, linearity, precision and accuracy, repeatability, sensitivity (LOD and LOQ) and robustness. The developed method is applicable for the determination of GLM in pure and different dosage forms with average recovery of 99.5 to 101.6% and the results are in good agreement with those obtained by the RP-HPLC reference method.

 

 

 

INTRODUCTION:

Glimepiride (GLM) belongs to sulfonylurea oral anti diabetic and GLM is an anti-diabetic drug which is used for the treatment of diabetes. GLM is a white to yellowish-white, odorless, crystalline powder insoluble in water. It is chemically described as 1-[[p-[2-(3-ethyl-4methyl-2-oxo-3-pyrroline-1carboxamido)ethyl] phenyl] sulfonyl]-3-(trans-4-methylcyclohexyl) urea (C₂₄H₃₄N₄O₅S) with a mol. mass of 490.62 g [1, 3], see Scheme 1.

 

 

 

 

Bromothymol blue C₂₇H₂₈Br₂O₅S (BTB), acts as a weak acid in solution. It can thus be in protonated or deprotonated form, appearing yellow or blue, respectively. It is bluish green in neutral solution. The deprotonating of the neutral form results in a highly conjugated structure, accounting for the difference in color. An intermediate of the deprotonating mechanism is responsible for the greenish color in neutral solution, mol. mass 624.40 g [4], see scheme 2. Bromothymol blue has been used as a reagent to form ion pair complex with drugs as diltiazem HCl [5].

 

 

Scheme 1: Chemical structure of Glimepiride, C₂₄H₃₄N₄O₅S (GLM).

 

Scheme 2: Chemical structure of bromothymol blue, C₂₇H₂₈Br₂O₅S (BTB).

 

A simple, accurate and sensitive two spectrophotometric methods were developed for the determination of glimepiride in pharmaceutical preparations. The first method was based on the formation of a charge-transfer complex of the drug, as n-electron donor, with 7,7,8,8-tetracyanoquinodimethane (TCNQ), as π-acceptor. The second method was based on the formation of ion-pair complexes between the examined drug and bromothymol blue (BTB). The proposed methods were validated for linearity, limit of detection, limit of quantification, precision, accuracy, robustness and specificity. The calibration was linear over the concentration range of 10–80 and 20–120 μg.mL^-1 for methods I and II, respectively. The limits of detection were 2.6 and 2.8 μg.mL^-1. The proposed methods were applied to the determination of the drug in pharmaceutical preparations [6].

 

A simple, sensitive, accurate and economical spectroscopic method has been developed for the estimation of glimepiride in bulk and in pharmaceutical dosage forms. An absorption maximum was found to be at 249 nm with the solvent system of chloroform. The drug follows Beer’s law limits in the range of 5-30 μg.mL^-1 with correlation coefficient of 0.999732. Results of the analysis were validated for accuracy, precision, LOD were found to be satisfactory. The proposed method is simple, rapid and suitable for the routine quality control analysis [7].

 

 Various spectrophotometric methods [8-27] have been reported for the determination of Glimepiride in pure as well as in dosage forms. Most spectrophotometric methods employ extraction procedures. In this case, the extracted complexes were into an organic solvent, which is immiscible with water, and the concentration of the resulting complex in the organic phase is determined spectrophotometrically. The complex extraction technique has some difficulties and inaccuracies due to incomplete extraction or the formation of emulsions between the hydrocarbon solvent and the basic compound-containing solution. In response to the problems resulting from extraction of the complex, it is better to determine formed complex without extraction [28]. Also none of the methods reported in the literatures is based on the formation of complex between BTB and GLM.

 

In this study, extraction-free spectrophotometric method for determination of GLM through ion-pair complex formation with BTB was developed.

 

MATERIALS AND METHODS:

Instrumentation:

Spectrophotometric measurements were made in Spectro scan 80 DV UV-VIS spectrophotometr with 1 cm quartz cells. An ultrasonic processor model Powersonic 405 was used to sonicate the sample solutions. The diluter pipette model DIP-1 (Shimadzu), having 100 μl sample syringe and five continuously adjustable pipettes covering a volume range from 20 to 5000 μL (model Piptman P, Gilson). ^{Centrifuge (Centurion Scientific Ltd., Model: K2080-Manufactured in the United Kingdom)} was used for preparation of the experimental solutions. Sartorius TE64 electronic balance was used for weighing the samples.

 

Reagents and drugs:

Glimepiride (99.98%) was supplied by Chempi fine chemicals (India), Metformin HCl (99.0%) and Rosuvastatin Calcium (98.6%). Bromothymol blue (97%) of analytical grade, chloroform Sigma-Aldrich and Na₂CO₃ extra pure were from Merck. All solvents and reagents were analytical grade chemicals.

 

Tablet commercial formulations were used for the analysis of GLM. The pharmaceutical formulations subjected to the analytical procedure were:

 

(1) Amarium tablets, Racha Lab, Aleppo–Syria, each tablet contains 2 and 4 mg of GLM (Mfg. 5/2016, Exp. 5/2018, respectively).

(2) Amapiride tablets, Avenzor Pharmaceutical Industries, Damascus–Syria, each tablet contains 20 and 4 mg of GLM (Mfg. 7/2017, Exp. 7/2019, respectively).

 

Standard stock solutions:

1´10^-3 mol.L^-1 of pure GLM was prepared in chloroform. This solution was prepared by good mixing 12.27 mg of GLM with 0.05 g of Na₂CO₃, adding 0.1 mL H₂O, drying well in 105^oC, after that it was dissolved in chloroform into a volumetric flask (25 mL) and diluted up to mark with chloroform. The solution was stored in dark bottles and kept in the refrigerator for not more than a week. The stock solution was further diluted daily just before the use to obtain working solutions of GLM in the concentration range 5.0-100 μM (2.453-49.062 μg.mL^-1 of GLM).

 

 

Stock standard solution of Bromophenol blue (BTB): 1x10^-2 mol/l:

Accurately weighed 160.93 mg of BTB was dissolved in chloroform into a volumetric flask (25 mL) and diluted up to mark with chloroform.

 

Recommended Procedure:

Aliquots of 1´10^-3 mol.L^-1 GLM solution (50, 75, 100, 200, 300, 400, 500, 600, 750. 850 and 1000 μL) containing 5, 7.5, 10, 20, 30, 40, 50, 60, 75, 85 and 100 μM (2.453, 3.680, 4.906, 9.812, 14.719, 19.625, 24.531, 29.437, 36.797, 41.703, and 49.062 μg.mL^-1) of GLM were transferred into a series of 10 mL calibrated volumetric flasks. Then 0.500 mL of BTB solution (1x10^-2 mol.L^-1) was added. The volume was made up to the mark with solvent and the absorbance was measured at λ_max 412nm in chloroform, against a similar reagent blank.

 

Procedure for pharmaceutical formulations:

Twenty tablets of each studied pharmaceutical formulation were weighed accurately, finely powdered and mixed well. An amount of the powder equivalent to the weight of one tablet was mixed well with 0.05 g of Na₂CO₃, adding 0.1 ml H₂O , drying well in 105^oC, and solved in chloroform using ultrasonic for 15 min, 10 ml of chloroform was added, filtered over a 10 ml flask and washed by the same solvent, then diluted to 10 ml with chloroform. This solution contains the following: 200 and 400 μg.mL^-1 of GLM for all studied pharmaceutical formulations contain 2 and 4 mg/tab, respectively.

 

Five solutions were prepared daily by diluting 1.0 mL from each stock solution of pharmaceutical formulations for contents: 2 or 4 mg/tab, then 0.4 ml from stock standard solution of BTB was added and adjusted the volume up to 10 ml with chloroform (these solutions contain 20 or 40 μg.mL^-1 of GLM respectively and  4x10^-4 mol.L^-1 of BTB; test solutions).

 

RESULTS AND DISCUSSION:

The effect of solvent:

The effect of the solvents (acetone, acetonitrile, dichloroethane, dichloromethane, chloroform and ethylacetate) on absorbance of reagent (BTB), formed complex [GLM][BTB] and the difference between them. It was found that chloroform solvent was the best, see Figure 1.

 

Fig. 1: Absorbance of reagent (BTB), formed complex [GLM]:[BTB] and difference between them (C_BTB 2.5x10^-4 M, C_Complex 5x10^-5 M, Blank is solvent, ℓ =1 cm).

 

Absorption Spectra:

UV-Vis spectra of GLM, BTB and the formed complex GLM:BTB solutions in chloroform was obtained. GLM solutions do not absorb in the range 300-600 nm. BTB solutions have small absorption at λ_max 412 ^{nm (e≈221} L mol^-1 cm^{-1 in chloroform)}. [GLM]:[BTB] complex solutions have maximum absorption at λ_max 412 nm in chloroform, e for the complex was 18024 ^{L mol-1 cm-1,} see Figure 2 as example.

 

 

 

Optimization of Variables:

The different experimental parameters affecting the spectrophotometric determination of GLM through ion-pair complex [GLM]:[BTB] formation with BTB in chloroform was studied in order to determine the optimal conditions for the determination of GLM.

 

The effect of time and temperature:

The effect of time and temperature on the complex [GLM]:[BTB] formation was studied within the ranges 5-120 min and ^{15-60o C.} It was found that the formed complex wasn't affected by time or temperature at those ranges.

 

 

Fig.2. UV-Vis spectra in chloroform of: 1- 0.8x10^-4 mol.L^-1 of GLM; 2- 5.0x10^-4 mol.L^-1 of BTB; 3- 0.5x10^-4 mol.L^-1 ion-pair complex (0.5x10^-4 mol.L^-1 of GLM with 5x10^-4 mol.L^-1of BTB ); Blank is 5.0x10^-4 mol.L^-1of BTB, 4- 0.5x10^-4 mol.L^-1 ion-pair complex (0.5x10^-4 mol.L^-1of GLM with 5x10^-4 mol.L^-1 of BTB ); Blank is chloroform, ℓ =1 cm.

 

The effect of BTB concentration:

The effect of BTB concentration on complex [GLM]:[BTB] formation was investigated. It was observed that the ^{absorbance of the formed complex increased coinciding with increasing the ratio of C}_BTB:C_GLM until the ratio (5:1), then slowly increased until the absorbance became a quasi-static at ratio more than 10.

 

Stoichiometric Relationship:

The molar ratio method:

The composition of GLM:BTB complex were determined by the molar ratio method and Job's method of continuous variation [29]. The stoichiometry of GLM:BTB complex was studied by molar ratio method according to following equation: A_max= f ([BTB]/[GLM]) at λ_max 412 nm in chloroform. It confirmed that the binding ratio of GLM:BTB complexes are equal to (1:1); where the concentration of GLM was constant (50 μM) and the concentrations of BTB changed from 0 to 250 μM, see Figure 3. The formation constant of the ion pair complex [GLM]:[BTB] is 3.1x10⁷ in chloroform.

 

The Job's method:

Continuous variation was utilized to check the composition of GLM:BTB complex at λ_max 412 nm in chloroform. The absorbance of the complex in used solvent were plotted against the mole fraction [BTB]/([GLM]+[BTB]), where [GLM]+[BTB]=100 µM. The plot reached maximum value at a mole fraction of 0.5, see Figure 4. This indicated complex formation (GLM:BTB) in the ratio of (1:1). The formation constant of the ion- pair complex [GLM]:[BTB] is 2.44x10⁷.

 

 

Fig.3. Molar ratio method to calculate binding ratio of GLM:BTB complex at λ=412 nm in chloroform ([GLM]= 50 μM, blank is chloroform, ℓ =1 cm).

 

 

Fig.4. Job's method of continuous variation to calculate binding ratio of GLM:BTB complex at λ 412 nm in chloroform ([GLM]+[BTB]=100μM, blank is chloroform, ℓ =1 cm).

 

Mechanism of reaction:

Anionic dyes such as BTB form ion-pair complexes with the positively charged nitrogen-containing molecule. The colour of such dyes is due to the opening of lactoid ring and subsequent formation of quinoid group (deprotonated). Glimepiride (C₂₄H₃₄N₄O₅S) is reacted with Na₂CO₃ to give (C₂₄H₃₃N₄H⁺O₅NaS), then dissolved in chloroform and forms yellow ion-pair complex with the dye at pH<3.8; (in pH>5.4 and alkaline solution BTB gives blue colour). Each drug-dye complex with two oppositely charged ions (positive on the drug and negative on the dye) behaves as a single unit held together by an electrostatic binding [13, 23-26]. The suggested mechanism of GLM-BTB ion-pair complex formation is shown in Scheme 3.

 

 

 

 

Scheme 3: The possible reaction mechanism of [GLM]:[BTB] complex formation.

 

Calibration curve:

The calibration curve of GLM in pure form through complexation with BTB showed excellent linearity over concentration range of 2.453-49.062 μg.mL^-1 in presence of 5.0´10^-4 mol.L^-1 of BTB with good correlation coefficient (R²= 0.9995) in chloroform. Regression ^{equation at λmax was as the follows:} y=0.0368x +0.0002 in chloroform. Figures 5 showed the spectra of [GLM]:[BTB] complex in presence of       5.0×10^-4 M of BTB as example. The spectra characteristics of the method ^{such as} the molar absorptivity ^{(ε), Beer's law,} regression equation at ^{λmax (y=a.x+b); where y=absorbance, a=slope, x=concentration of GLM by μg.mL-1, b=intercept, the correlation coefficient,} limit of detection (LOD) and limit of quantification (LOQ) and the optimum conditions for spectrophotometric determination of GLM through ion-pair complex formation using BTB in chloroform ^{is summarized in Table 1.}

 

Fig.5. Spectra of [GLM]:[BTB] complex in presence of 5.0´10^-4 M of BTB; where C_GLM as the follows: 2.453, 3.68, 4.906, 9.812, 14.719, 19.625, 24.531, 29.437, 36.797, 41.703 and 49.062 μg.mL^-1 {Blank is BTB solution in chloroform 5.0´10^-4 M; ℓ = 1cm}.

 

 

Fig. 6. Calibration curve for determination of GLM according to optimal conditions at λmax 412 nm (in present of 5.0´10^-4 M of BTB) where GLM: 2.453 - 49.062 μg/ml {Blank is BTB solution in chloroform 5.0´10^-4 M ; ℓ = 1 cm}.

 

Analytical results:

Spectrophotometric determination of GLM through complexation with BTB in chloroform within optimal conditions using calibration curve was applied. The results, summarized in Table 1, showed that the determined concentration of GLM was rectilinear over the range of 2.453 to 49.062 μg.mL^-1, with relative standard deviation (RSD) not more than 3.8%. The results obtained from the developed method have been compared with the official RP-HPLC method [30] and good agreement was observed between them.

 

 

 

 

Table 1. The parameters established for spectrophotometric determination of GLM by complex formation with BTB in chloroform.

parameters	Operating values
λmax of GLM:BTB complex, nm	412
Beer’s Law Limit, for CGLM by μM	5-100
Beer’s Law Limit, for CGLM by μg.mL-1	2.453 - 49.062
Molar absorptivity of [GLM]:[BTB] complex (ε) L.mol-1.cm-1	18024
Regression equation for [GLM]:[BTB] at λmax=412 nm:
Slope	0.0368
Intercept	0.0002
Correlation coefficient (R2)	0.9995
LOD for CGLM by μg.mL-1 in [GLM]:[BTB]	0.30
LOQ for CGLM by μg.mL-1	0.91
RSD%	3.8
CBTB:CGLM, M	≥5
Stability	10 h
Temperature of solution	20±5oC

 n=5, t=2.776.

 

Applications:

The developed spectrophotometric method was applied to determine GLM in some pharmaceutical preparations through complex formation by BTB in chloroform according to the optimal conditions. The results of quantitative analysis for GLM in pharmaceutical preparations were summarized in Table 2. The proposed method was simple, direct, specific and successfully applied to the determination of GLM in pharmaceuticals without any interference from excipients. Average recovery ranged between 98.6 to 101.8%. The results obtained by this method agree well with the contents stated on the labels and were validated by RP-HPLC method [30].

 

Table 2. Determination of GLM, in some Syrian pharmaceutical preparations using ^{spectrophotometric method through} complex formation with BTB in chloroform, λ_max 412 nm

Tablet dosage form	Label Claim of GLM, mg/tab.	*Mean ±SD (GLM), mg/tab.	RSD%	Assay %	*Mean ±SD (GLM), mg/tab. by RP-HPLC[30]	* Assay %, by RP-HPLC[30]
Amarium	2	1.999±0.070	3.5	99.9	2.003±0.032	100.3
	4	3.965±0.119	3.0	99.1	3.984±0.053	99.6
Amapiride	2	2.036±0.069	3.4	101.8	2.038±0.037	101.9
	4	3.945±0.122	3.1	98.6	3.960±0.052	99.0

* n=5.

 

METHOD VALIDATION:

The developed method for simultaneous estimation of GLM has been validated in accordance with the International Conference on Harmonization guidelines (ICH) [31].

 

Selectivity:

Selectivity test determines the effect of excipients on the assay result. To determine the selectivity of the method, standard solution of GLM, commercial product solution and blank solutions were analyzed. The results of the tests proved that the components other than the drug did not produce any interfere.

Linearity:

Several aliquots of standard stock solution of GLM were taken in different 10 ml volumetric flask and diluted up to the mark with chloroform such that their final concentrations were 2.453 - 49.062 μg.mL^-1 for GLM. Absorbance was plotted against the corresponding concentrations to obtain the calibration graph, see Figure 5 and Table 3. Linearity equations obtained were y = 0.0368x + 0.0002 for the range 2.453 - 49.062 μg.mL^-1 (R²=0.9995).

 

 

 

^{Table 3. Spectrophotometric determination of GLM through complex formation with BTB within optimal conditions using calibration curve in chloroform}_.

 * n=5, t= 2.776.

 

Precision and Accuracy:

The precision and accuracy of proposed method was checked by recovery study by addition of standard drug solution to pre-analyzed sample solution at three different concentration levels (80%, 100% and 120%) within the range of linearity for GLM. The basic concentration level of sample solution selected for spiking of the GLM standard solution was 14.719 μg.mL^-1. The proposed method was validated statistically and through recovery studies, and was successfully applied for the determination of GLM in pure and dosage forms with percent recoveries ranged from 99.4% to 101.8%, see Table 4.

 

Table 4. Results of recovery studies (n=5).

Level	% Recovery
80%	99.4
100%	101.8
120%	101.0

 

Repeatability:

The repeatability was evaluated by performing 10 repeat measurements for 14.719 μg.mL^-1 of GLM using the studied spectrophotometric method under the optimum _{conditions. The found amount of GLM (± SD)} 14.940 ± _0.42 μg.mL^-1 _{and the} percentage recovery was found to be 101.5 ± 2.8 with RSD of 0.042. These values indicate that the proposed method has high repeatability for GLM analysis.

 

Sensitivity (LOD and LOQ):

The sensitivity of the method was evaluated by determining the LOD and LOQ. The values of LOD and LOQ for GLM are 0.30 and 0.91 μg.mL^-1, respectively.

 

Robustness:

The robustness of the method adopted is demonstrated by the constancy of the absorbance with the deliberated minor change in the experimental parameters such as the change in the concentration of excipients, C_BTB:C_GLM (10±5%), temperature (20±5^oC), stability (10±5% h) and reaction time (5±1 min), see Table 5 which indicates the robustness of the proposed method. The absorbance was measured and assay was calculated for five times.

 

Specificity:

The specificity of the method was ascertained by analyzing standard GLM in presence of excipients. There was no interference from most of the common excipients.

 

Table 5. Robustness of the proposed spectrophotometric method.

Experimental parameter variation	Average recovery (%)*
	CGLM
	4.906μg/ml	36.797μg/ml
Temperature 15oC 25oC	99.3 99.6	99.9 101.16
Stability 9.5 h 10.5 h	99.8 100.6	100.3 100.2
Reaction time 4.0 min 6.0 min	99.8 100.2	99.9 100.4

 ^{* n=5.}

 

The homogenization of tablets:

The homogenization of tablets in terms of the weight and the amount of drug was studied. We found that the mean weight and amount drug in the tablets was 0.1793 ± 0.0030 g (i.e. ±1.67%), 0.1794 ± 0.0023 g (i.e. ±1.17%) for Amarium tablets (2 and 4 mg/tab) and 0.1005 ± 0.0015g (i.e. ±1.49%) and 0.0993 ± 0.0012 g (i.e. ±1.21%) Amapiride tablets (2 and 4 mg/tab), respectively. While the mean amount drug in the tablets was ^{1.999±0.070 mg/tab} (i.e. ±3.5%) and ^{3.965±0.119 mg/tab} (i.e. ±3.0%) for Amarium tablets (2 and 4 mg/tab) and ^{2.036±0.069 mg/tab} (i.e. ±3.4%) and ^{3.945±0.122 mg/tab} (i.e. ±3.1%) for Amapiride tablets (2 and 4 mg/tab), respectively; which shows that homogeneity of tablets is good.

 

Interferences:

Metformin HCL up to 1000 mg with 2 mg of glimepiride does not interfere, but Pioglitazone HCL and Rosuvastatin Calcium interfere.

 

CONCLUSION:

The developed spectrophotometric method is simple, direct (extraction-free) and cost-effective for the determination of GLM in pure and tablet dosage forms. This method is based on formation of ion-pair complex between GLM and BTB in chloroform ([GLM]:[BTB]. Beer’s law in the optimum experimental conditions using [GLM]:[BTB] complex is valid within a concentration range of 2.453 - 49.062 μg/ml. The developed method is applied for the determination of GLM in pure and its commercial tablets without any interference from excipients with average recovery of 99.4 to 101.8%.

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31.   ICH: Proceedings of the International Conference on Harmonization of Technical Requirement of Registration of Pharmaceuticals for Human Use (ICH Harmonized Tripartite Guidelines).

 

 

Accepted on 04.05.2018           © RJPT All right reserved

Research J. Pharm. and Tech 2018; 11(7): 3049-3056.