Simultaneous Estimation of Metoprolol and Atorvastatin in Combined   Dosage Form

 

V Niraimathi*, V Prema , Ajithadas Aruna ad A Jerad Suresh

Department of  Pharmaceutical Chemistry, College of Pharmacy, Madras Medical College, Chennai-600 003, Tamil Nadu, India.

*Corresponding Author E-mail:  vnm_anr@yahoo.co.in

 

ABSTRACT:

Three novel, simple, sensitive, economical, accurate, rapid and precise spectrophotometric methods have been developed for the simultaneous estimation of Metoprolol and Atorvastatin in pharmaceutical oral solid dosage form. Method A involves solving of simultaneous equations based on measurement of absorbance at two wavelengths 223nm and 241nm, λmax of metoprolol and atorvastatin respectively. Method B involves estimation by Absorbance ratio method at 231 nm ( iso absorptive point ) and at 241 nm ( λmax of atorvastatin ). Method C involves AUC for the normal spectrum. All the methods utilize methanol as solvent.  Beer’s  law  was obeyed in the concentration  range of 5-90 µg/mL for both the drugs  and the linearity was established. The results of analysis have been validated statistically and by recovery   studies. The proposed methods can be successfully  applied for the estimation of metoprolol and atorvastatin in combined dosage form.

 

KEYWORDS: Beer’s law, Simultaneous equation, Absorbance ratio , Area under curve (AUC), Metoprolol (METO) and Atorvastatin (ATOR).

 


INTRODUCTION

Metoprolol is a selective beta 1-adrenoceptor blocking agent. It is given as succinate and  used for treatment of hypertension. It is chemically 1-[4-(2-methoxyethyl) phenoxy]-3-(propan-2-yl amino) propan-2-ol. It is official in USP. Atorvastatin is a lipid lowering agent and is given as calcium salt. It is an inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase. This enzyme catalyzes the conversion of HMG-CoA to mevalonate, an early and rate-limiting step in cholesterol biosynthesis1-2. It is chemically [R-(R*, R*)]-2-(4-fluorophenyl)-β, δ- dihydroxy-5-(1-methylethyl)-3-phenyl-4-[(phenylamino)carbonyl]-1Hpyrrole-1-ptanoic acid, calcium salt (2:1) trihydrate. Combination of these two is used for the treatment of hypertension. Literature survey reveals availability of UV, HPLC and  HPTLC methods for estimation of METO and ATOR individually with many drugs other than this particular combination3-9. However no method has been reported till date for the estimation of METO and ATOR in combined dosage form. Present work deals with the quantitative estimation of METO and ATOR in their combined dosage form by various UV spectrophotometric methods.

 

EXPERIMENTAL:

Instrumentation:

All spectral measurements were made on shimadzu UV-VIS spectrophotometer – 1650 with 1 cm matched quartz cells.

 

Preparation of  standard stock solution:

An accurately  weighed  quantity of  50mg of  metroprolol and atorvastatin were separately taken in a 50ml volumetric flask, dissolved in methanol and made up to volume using the same to get 1mg /mL.

 

Preparation of sample solution:

The average net content of  20 capsules were determined and the powder equivalent to 23.75mg of  meto was taken in 50 mL volumetric flask and dissolved in methanol and made up to  the volume with the same. The solution was then filtered using whatman filter paper, the first few mL of the filtrate was discarded  and the remaining solution was used for further analysis.

 

ASSAY PROCEDURE:

Method A - Simultaneous equation method:8-9

Aliquots of the standard  stock solutions were transferred to a series of 50mL  volumetric    flask  and  suitably  diluted with  methanol to give varying concentrations ranging from 5 - 50 µg/mL for both METO and ATOR. The  solutions  of  METO  and ATOR were scanned in the range of 200 to 400 nm against distilled water as blank.Two wavelengths selected  for  simultaneous  equation  method  were  223 nm  and  241nm. Absorbance and absorptivities of series of standard solutions were recorded at selected wavelengths  λ1  and   λ2 .  Two  simultaneous  equations  were  formed  using  these absorptivity  coefficient values

 

A1 = 367.25 C1 + 9.044 C2--------------------------- (1)

A2 = 331.87 C1 + 288.01 C2-------------------------- (2)

 

Where,    A1 and A2   were absorbances of sample solution at 223 nm and 241 nm respectively. C1 and C2   are the concentrations of ator and meto respectively in sample solution in g / L. Aliquots of sample solution were diluted suitably and absorbances of the  final  dilution  were  measured  at  223 nm  and  241  nm  and concentration of two drugs in sample was calculated using above framed simultaneous equations – (1) & (2).

 

Results of analysis are reported in Table-2. Validity of  above  framed   equation  was checked by repeating  the  procedure  with  different  standards  of  METO  and ATOR, measuring  their  absorbance at respective wavelengths and calculating concentration of two  components. The results of validation studies was found satisfactory. The analysis procedure was repeated five times with the formulation.

 

Method B - Absorbance  Ratio method:10-12

Absorbance ratio method (Q-anaysis) uses the ratio of  absorbances  at  two selected wavelengths, one at isoabsorptive point and other being the λmax of one of the two components.  Aliquots of the standard stock solutions were transferred to a  series of 50mL volumetric flask and suitably diluted to  give  varying  concentrations  ranging from 5 - 50 µg/mL for METO  and  ATOR.. The  solutions of meto and ATOR were scanned in the range of 200 to 400 nm against  distilled  water  as  blank.  The  overlain spectra of metoprolol and  atorvastatin  was  taken  and  isoabsorptive  wavelength  was found  to be  at  231 nm (fig-1).  From  the  overlain  spectra,  the  wavelengths  231 nm ( isoabsorptive point ) and 241nm ( λmax of atorvastatin ) were  selected for formation of  Q – absorbance equation. The absorptivity  values of each drug  at  isoabsorptive point  were  determined. The  concentration  of   each  drug  in  the  combination  was determined  by  substituting  the  absorbance and  absorptivity  values in  the  following equations:

 

CX  = ( Qm-Qy / Qx-Qy )  x ( A / ax1 )

CY  = ( Qm-Qx / Qy-Qx )  x ( A / ay1 )

Where, Cx = concentration of  ATOR, Cy= concentration of  METO, Qm= ratio of absorbance of sample at selected wavelengths, Qx= ratio of absorptivity of ATOR, Qy= ratio of absorptivity of METO, ay1= absorptivity of METO at 231 nm, ax1= absorptivity of ATOR at 231 nm.

 

Results of analysis of capsule formulation are reported in Table-2.

 

Method C -  Area under curve method:13

The mixed standard solutions of meto and ator were scanned between 200-400 nm using distilled water as blank. The  area under  curve between 215-235 nm for meto and 240-295 nm for atorvastatin  were measured  by  using  the  inbuilt  software.  The inbuilt software calculates the area  bound  by  the  curve and  the  horizontal  axis.  The horizontal axis is selected by entering the wavelength range over which the area  has  to be calculated .The wavelength range is selected on the  basis  of  repeated  observations so as  to get  the linearity between area under curve  and  concentration. The amount of meto  and  ator in  marketed  sample  was  computed  from  the  calibration  curve obtained by plotting the area versus concentration. Results of analysis  are reported .

 

Fig(1)

 

RECOVERY STUDIES:

To study the accuracy and reproducibility of the proposed methods, recovery experiments were carried out by adding a known amount of standard drug solution of METO and ATOR to preanalysed sample and the percentage recovery is calculated. The results are furnished in table-2.

 

RESULTS AND DISCUSSION:

The optical characteristics such as absorption maxima, Beer's law limits, molar absorptivity and Sandell's sensitivity are furnished in Table-1. The regression characteristics like slope, intercept, and correlation co-efficient (r), obtained from different concentrations were calculated and the results are summarized in Table-1.

 

The results indicate that there is no interference of other ingredients present in the formulation. Thus, the proposed methods are simple, sensitive, precise, accurate, rapid, economic, reproducible and useful for the simultaneous determination of meto and ator in pharmaceutical formulation. The value of standard deviation & correlation coefficient were satisfactory and recovery studies were indicative of the accuracy of the proposed methods in method A, two wavelengths of respective absorbance maxima ie 223nm for METO and 241nm for ATOR were used for the analysis of the durgs. The criteria for obtaining maximum Precision14 by this method were calculated and found to be outside the range of 0.1-2. In method B,  the primary requirement for developing a method for analysis is that the entire spectra should follow the Beer’s  law at all wavelengths which was fulfilled in case of both these drugs.


Table 1: Optical & statistical parameters

Parameters

Simultaneous equation

Absorbance ratio method

Area under curve

METO

ATOR

METO-223

ATOR-241

METO-231

ATOR-231

METO

ATOR

λmax (nm)

223

241

223

241

231

231

215-235

240-295

Beer’s law limits (µg/mL)

5-50

5-50

5-50

5-50

5-50

5-50

5-50

5-50

Molar absorptivity(L mol-1 cm-1)

18801

44415

18801

44415

8286.93

41408

-

-

Sandell’s sensitivity (µg cm-2 /0.001 abs  unit)

0.034776

0.0272

0.03477

0.0272

0.07899

0.0292

-

-

Slope (m)

0.027663

0.0363

0.02766

0.0363

0.01199

0.0337

0.11805

0.31599

Intercept(c)

0.013067

0.0051

0.01306

0.0051

0.00826

0.0065

0.0702

-0.0424

Regression equation*

0.02766X

+0.013

0.0363X

+0.0051

0.02766X

+0.013

0.0363X

+0.0051

0.01199X

+0.008

0.0337X

+0.0065

0.011805X+0.07

0.31599X-0.042

Correlation coefficient (r)

0.9997

0.9999

0.9997

0.9999

0.9993

0.9999

0.9996

0.9994

Standard deviation

0.026133

0.01033

0.02613

0.0103

0.01653

0.0129

0.1404

-0.0848

% RSD**

0.1099

0.1036

0.1099

0.1036

0.06962

0.12991

0.29231

-0.84378

LOD

3.1175

0.92143

3.1175

0.9214

4.54883

1.2666

3.92492

-0.88561

LOQ

9.4471

2.7922

9.4471

2.7922

13.7843

3.8381

11.8937

-2.68367

* (Y=mx+c), ** mean of three readings

 

Table – 2: Assay and recovery of METO and ATOR  in dosage forms

Method

Drug

Label claim (mg)

Amount  obtained(mg)*

% Label claim

Percentage recovery **

Simultaneous equation

 

METO

23.75

23.77

100.1

99.00

ATOR

10

9.97

99.7

100.17

Absorbance ratio method

 

METO

23.75

23.74

99.95

99.73

ATOR

10

9.93

99.3

99.65

Area under curve

 

METO

23.75

23.82

100.29

99.82

ATOR

10

10.05

100.50

100.95

* Average of three determinations, ** Average of three determinations

 


The two wavelengths used for the analysis of both drugs were 231nm (isoabsorptive point) and 241nm (λmax of ATOR) The validation parameters were studied at all the three wavelengths for both the methods. Accuracy was determined by calculating the % recovery and precision was calculated as repeatability (standard deviation & RSD) and inter & intra day variation for both the drugs. Both the methods were successfully used to determine the amounts of METO and ATOR present in the formulation. The results obtained are in well agreement with the corresponding labeled amount.

 

CONCLUSION:

The methods described in this paper for the simultaneous estimation of METO and ATOR are found to be simple, sensitive, economical, accurate, rapid and precise. Hence these three methods could be successfully employed for the routine analysis of METO and ATOR in their combined dosage form.

 

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Received on 22.12.2009       Modified on 17.02.2010

Accepted on 21.03.2010      © RJPT All right reserved

Research J. Pharm. and Tech. 3(2): April- June 2010; Page 586-588