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Pharmaceutica Analytica Acta
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Simple and Sensitive Spectrophotmetric Determination of Lamotrigine in Pure Form and in Dosage Forms

Rajendraprasad N1*, Basavaiah K2, Vinay KB2 and Ramesh PJ2

1Department of Chemistry, J.S.S. College, Ooty Road, Mysore-570025, Karnataka, India

2Department of Studies in Chemistry, Manasagangothri, University of Mysore, Mysore-560 006, India

*Corresponding Author:
Rajendraprasad N
Department of Chemistry
J.S.S. College, Ooty Road
Mysore-570025, Karnataka, India
E-mail:[email protected]

Received date: November 15, 2012; Accepted date: November 21, 2012; Published date: November 25, 2012

Citation: Rajendraprasad N, Basavaiah K, Vinay KB, Ramesh PJ (2012) Simple and Sensitive Spectrophotmetric Determination of Lamotrigine in Pure Form and in Dosage Forms. Pharmaceut Anal Acta 3:188. doi: 10.4172/2153-2435.1000188

Copyright: © 2012 Rajendraprasad N, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Two new simple, sensitive and cost-effective spectrophotometric methods for the determination of lamotrigine (LMT) in bulk drug and in tablets are described. The methods are based on the measurement of absorbance of LMT either in 0.1 M H2SO4 (method A) or in methanol (method B) at 225 nm. Linearity was found to be in the ranges, 0.5- 5.0 and 1.25-12.5 μgmL-1 LMT, for method A and method B, respectively with apparent molar absorptivity values of 8.65×104 and 2.11×104 l mol-1cm-1. The Sandell sensitivity values, limits of detection (LOD) and quantification (LOQ) values have also been reported for both the methods. The accuracy and precision of the methods were evaluated on intra-day and inter-day basis; the relative error (%RE) and the relative standard deviation (RSD) were <2.0%. The proposed methods were applied to the determination of the examined drug in coated tablet and no interference from any common pharmaceutical additives and diluents was observed. Results of assay were validated statistically by parallel analysis and by recovery studies.

Keywords

Spectrophotometry; Determination; Lamotrigine; Pharmaceutical formulations

Introduction

Lamotrigine (LMT), [6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5- diamine], is an anticonvulsant drug and has been used as antiepileptic to treat epilepsy and bipolar disorder as monotherapy and as an adjunct with other antiepileptics for treatment of partial and generalized toxicchronic seizures. It is also used to treat neurological lesions and as a tranquilizer [1,2]. Its chemical structure is given in figure 1.

pharmaceutica-analytica-acta-Chemical-structure-LMT

Figure 1: Chemical structure of LMT.

LMT is not official in any pharmacopoeia. Chromatographic techniques have been widely employed for the determination of LMT in body fluids. Published methods for the determination of LMT in biological samples include high-performance liquid chromatography (HPLC) [3-10], high-performance thin layer chromatography (HPTLC) [11] and gas-chromatography (GC) [12] and for assay in pharmaceuticals include planar chromatography [13], TLC and HPLC [14], HPLC and GC [15] and capillary electrophoresis [16,17] have been reported. Two immunoassay techniques [18,19] have also been developed for determination of LMT in biological samples. Few methods have been reported for its determination in pharmaceuticals and include titrimetry [20] with acetous perchloric acid, in anhydrous acetic acid medium, UV- spectrophotometry [21] and visible spectrophotometry [22-25]. The uv-spectrophotometric method [21] was used for determination of LMT in tablets, where the tablet extract in 0.1 M NaOH was measured at 305 nm. Youssef and Taha [14] have reported the application of visible spectrophotometry for the determination of LMT using chloranilic acid as a chromogen. The reported method is less sensitive with a linear range 10-200 μgmL-1 and the molar absorptivity of 1.28×103 mol-1cm-1. Though the method is claimed to be selective, any N-containing basic moiety would definitely interfere with the assay. The extraction spectrophotometric methods [22-25] are at the other hand involves tedious extraction steps and consumes longer time for the analysis.

Many of the other reported methods [13-17] are sensitive and selective but they are time consuming, require expensive instrumental setup, and some require preliminary sample treatment. Adsorptive stripping voltammetric method [26] is highly complicated and is reported to be less precise (RSD, ~10%). Considering these drawbacks, there was a need to develop more advantageous spectrophotometric method for its determination in bulk powder and commercial dosage forms.

The objective of this investigation is to develop new simple, fast, sensitive, selective, reliable and inexpensive UV Spectrophotometric methods for the determination of LMT in bulk drug and commercial pharmaceutical formulations. The methods are based on the measurement of absorbance of LMT solution in either 0.1 M H2SO4 or methanol at 225 nm.

Experimental

Apparatus

The Spectrophotometric measurements were carried out using Shimadzu Pharmaspec 1700 UV/Visible spectrophotometer.

Chemicals and reagents

All chemicals used were of analytical reagent grade.

Pure LMT (pharmaceutical grade, 99.88%) sample was kindly provided by Cipla India Ltd, Mumbai, India, as a gift and used as received. Commercial dosage forms used: lamosyn 100 and lamosyn 25 (both from Sun Pharmaceuticals Ltd, Mumbai, India) and Lametec 50- DT (Cipla India Ltd, Mumbai, India)-all tablets were purchased from local commercial sources.

Standard solutions

Sulphuric acid (0.1 M) was prepared by successive dilutions of appropriate volume of concentrated acid (S.D. Fine Chem, Mumbai, India, sp. gr. 1.84) in water. Methanol AR (S.D. Fine Chem, Mumbai, India) was used as solvent in the present study.

Standard drug solution

Standard drug solutions of 10 μgmL-1 in 0.1 M H2SO4 and 25 μgmL-1 LMT in methanol were prepared separately and used for assay in method A and method B, respectively.

Procedures

Recommended procedure and calibration curve

Method A: Varying amounts of aliquots (0.5, 1.0, 2,0, 3.0, 4.0 and 5.0 mL) of working standard solution corresponding to 0.5-5.0 μgmL-1 LMT were taken into a series of 10 mL volumetric flasks and volume was made upto mark with 0.1 M H2SO4. The absorbance of each solution was measured at 225 nm vs. 0.1 M H2SO4.

Method B: Into a series of 10 mL calibration flasks, aliquots of lamotrigine standard solution (25 μgmL-1) equivalent to 1.25-12.5 μgmL-1 LMT were accurately transferred and volume was made upto mark with methanol. The absorbance of each solution was measured at 225 nm vs. methanol.

In both the cases, calibration curves were plotted and the concentration of the unknown was read from the calibration graph or computed from the regression equation derived using Beer’s law data.

Procedure for tablets

Method A: Weighed amount of tablet powder equivalent to 10 mg of LMT was transferred into a 100 mL volumetric flask. The content was shaken well with about 50 mL of 0.1 M H2SO4 for 20 min. The mixture was diluted to the mark with the same acid. It was filtered using Whatman No 42 filter paper. First 10 mL portion of the filtrate was discarded and a subsequent portion was diluted to get a working concentration of 10 μgmL-1 and subjected to analysis following the procedure described earlier.

Method B: Tablet powder equivalent to 10 mg of LMT was transferred into a 100 mL volumetric flask. The content was shaken well with about 50 mL of methanol for 20 min and diluted to the mark with the same solvent. It was filtered using Whatman No 42 filter paper. First 10 mL portion of the filtrate was discarded and subsequent portion was analyzed after dilution to 25 μgmL-1 LMT with methanol.

Results and Discussion

Spectral characteristics

The LMT was dissolved either in 0.1 M H2SO4 (method A) or methanol (method B) and the absorbance measured at 225 nm, and at this wavelength blank solution had insignificant absorbance as shown by the absorption spectra in figure 2.

pharmaceutica-analytica-acta-UV-absorption-spectra

Figure 2: UV absorption spectra of LMT.

Method validation

Linearity, sensitivity, limits of detection and quantification: A linear correlation was found between absorbance at λmax and concentration of LMT in the ranges given in table 1. The graphs are described by the regression equation:

Parameter Method A Method B
λmax, nm 225 225
Linear range, µg mL-1 0.5-5.0 1.25-12.5
Molar absorptivity(ε), L mol-1cm-1 8.65×104 2.11×104
Sandell sensitivity*, µg cm-2 0.003 0.0121
Limit of detection (LOD), mgmL-1 0.01 0.03
Limit of quantification (LOQ), mgmL-1 0.02 0.09
Regression equation, Y**
Intercept (a) 0.0161 -0.0094
Slope (b) 0.3311 0.0849
Standard deviation of a (Sa) 0.0998 0.0998
Standard deviation of b (Sb) 0.032 0.0124
Variance (Sa2) 0.01 0.01
Regression coefficient (r) 0.9999 0.9999

Table 1: Sensitivity and regression parameters.

Y=a+bX (Where Y=absorbance of 1 cm layer of solution; a=intercept; b=slope and X=concentration in μgmL-1). Regression analysis of the Beer’s law data using the method of least squares was made to evaluate the slope (b), intercept (a) and correlation coefficient (r) for each system and the values are presented in Table 1. A plot of log absorbance vs. log concentration, yielded straight lines with slope equal to 0.991 and 1.02 for method A and method B, respectively, further establishing the linear relation between the two variables. The optical characteristics such as Beer’s law limits, molar absorptivity and Sandell sensitivity values [27] of all the three methods are also given in table 1. The limits of detection (LOD) and quantitation (LOQ) calculated according to ICH guidelines [28] using the formulae:

LOD=3.3 S/b and LOQ=10 S/b, (where S is the standard deviation of blank absorbance values, and b is the slope of the calibration plot) are also presented in table 1. The high values of ε and low values of Sandell sensitivity and LOD indicate the high sensitivity of the proposed methods.

Precision and accuracy: The assays described under “general procedures” were repeated seven times within the day to determine the repeatability (intra-day precision) and five times on different days to determine the intermediate precision (inter-day precision) of the methods. These assays were performed for three levels of analyte. The results of this study are summarized in table 2. The percentage relative standard deviation (%RSD) values were ≤ 1.96% (intra-day) and ≤ 1.99% (inter-day) indicating high precision of the methods. Accuracy was evaluated as percentage relative error (RE) between the measured mean concentrations and taken concentrations for LMT. Bias {bias %=[(Concentration found-known concentration)×100/known concentration]} was calculated at each concentration and these results are also presented in table 2. Percent relative error (%RE) values of ≤ 1.67% demonstrate the high accuracy of the proposed methods.

Method LMT taken,
µg mL-1
Intra-day accuracy and precision (n=7) Inter-day accuracy and precision  (n=5)
LMT found ± CL, 
µg mL-1
%RE %RSD LMT found ± CL, 
µg mL-1
%RE %RSD
A 1.0
3.0
5.0
0.99 ± 0.04
3.05 ± 0.12
4.98 ± 0.11
1.0
1.67
0.4
1.52
1.65
0.89
1.01 ± 0.03
3.03 ± 0.07
4.96 ± 0.14
1.0
1.0
0.8
0.98
0.86
1.02
B 4.0
8.0
12.0
4.05 ± 0.19
8.12 ± 0.24
12.02 ± 0.46
1.25
1.5
0.12
1.96
1.23
1.56
4.03 ± 0.17
7.98 ± 0.32
11.86 ± 0.65
0.75
0.25
1.17
1.55
1.46
1.99

Table 2: Evaluation of intra-day and inter-day accuracy and precision.

Selectivity: A systematic study was performed to determine the effect of matrix by analyzing the placebo blank and synthetic mixture containing LMT. A placebo blank of the composition: starch (10 mg), acacia (15 mg), hydroxyl cellulose (10 mg), sodium citrate (10 mg), talc (20 mg), magnesium stearate (15 mg) and sodium alginate (10 mg) was made and its solution was prepared as described under ‘tablets’, and then subjected to analysis. The absorbance of the placebo solution in each case was almost equal to the absorbance of the blank which revealed no interference. To assess the role of the inactive ingredients on the assay of LMT, a synthetic mixture was separately prepared by adding 10 mg of LMT to the placebo mentioned above. The drug was extracted and solution prepared as described under the general procedure for tablets. The solutions after appropriate dilution were analyzed following the recommended procedures. The absorbance resulting from 3 and 8 μgmL-1 LMT solution in method A and method B, respectively, had nearly the same as those obtained for pure LMT solutions of identical concentrations. This unequivocally demonstrated the non-interference of the inactive ingredients in the assay of LMT. Further, the slopes of the calibration plots prepared from the synthetic mixture solutions were about the same as those prepared from pure drug solutions.

Robustness and ruggedness: The robustness of the methods was evaluated by making small incremental changes in the concentration of H2SO4 in method A. The results obtained from the altered acid conditions were not different compared to the optimum conditions. Method ruggedness was demonstrated having the analysis done by four analysts, and also by a single analyst performing analysis on four different instruments in the same laboratory. Intermediate precision values (%RSD) in both instances were in the range 0.88-1.65% indicating acceptable ruggedness. The results are presented in table 3.

Method LMT taken,
µg mL-1
Robustness Ruggedness
Parameter altered* Inter-analysts, (%RSD)
(n=4)
Inter-instruments, (%RSD)
(n=4)
H2SO4 concentration, (%RSD)
A 6.0 1.56 0.88 1.65
B 8.0 - 0.99 1.58

Table 3: Method robustness and ruggedness expressed as intermediate precision (% RSD).

Analysis of pharmaceutical formulations: The proposed methods were applied for the quantification of LMT in commercial tablets. The results were compared with these obtained using a published method [14]. The method consisted of the measurement of the absorbance of the charge-transfer complex of LMT with p-chloranilic acid in acetone at 519 nm. Statistical analysis of the results did not detect any significant difference between the performance of the proposed methods and reference method with respect to accuracy and precision as revealed by the Student’s t-value and variance ratio F-value [29]. The results of assay are given in table 4.

Tablet brand
nameψ
Nominal amount,
 (mg/tablet)
Found* (Percent of label claim  ±  SD)

Reference method

Proposed methods
Method A Method B
Lamosyn-100a 100 101.3 ± 0.42 102.1 ± 1.04
t=1.73
F=6.13
100.8 ± 0.86
t=1.23
F=4.19
Lamosyn-25a 25 97.48 ± 0.62 98.72 ± 0.92
t=2.54
F=2.2
96.85 ± 1.48
t=0.77
F=5.70
Lametec-50 DTb 50 103.5 ± 0.72 104.1 ± 1.46
t=0.87
F=4.11
102.8 ± 1.62
t=0.95
F=5.06

Table 4: Results of analysis of tablets by the proposed methods and statistical comparison of the results with the reference method.

Recovery study: To further assess the accuracy of the methods, recovery experiments were performed by applying the standardaddition technique. The recovery was assessed by determining the agreement between the measured standard concentration and added known concentration to the sample. The test was done by spiking the pre-analysed tablet powder with pure LMT at three different levels (50, 100 and 150% of the content present in the tablet powder (taken) and the total was found by the proposed methods. Each test was repeated three times. In all the cases, the recovery percentage values ranged between 100.8 and 105.1% with relative standard deviation in the range 0.98-1.56%. Closeness of the results to 100% showed the fairly good accuracy of the methods. The results are shown in table 5.

Tablet studied Method A Method B
LMT in tablet, µg mL-1 Pure LMT added, µgmL-1 Total found, µg mL-1 Pure LMT recovered
(Percent ± SD*)
LMT in tablet, µgmL-1 Pure LMT added, µgmL-1 Total found, µgmL-1 Pure LMT recovered
(Percent ± SD*)
Lamosyn-100 2.04
2.04
2.04
1.0
2.0
3.0
3.077
4.142
5.097
103.7 ± 1.32
105.1 ± 1.08
101.9 ± 1.56
4.03
4.03
4.03
2.0
4.0
6.0
6.062
8.062
10.18
101.6 ± 1.32
100.8 ± 0.98
102.5 ± 1.16

Table 5: Results of recovery study via standard-addition method.

Conclusions

Two UV-spectrophotometric methods for the determination of lamotrigine in bulk drug and in pharmaceutical dosage forms were developed and validated for accuracy, precision, linearity, robustness and ruggedness. The proposed methods have better linear dynamic ranges and sensitivity compared to the reported uv [21] and visible [14] spectrophotometric methods (Table 6). The methods have the advantages of simplicity without involving heating or extraction step and high sensitivity. No interference due to co-formulated substances was observed when applied to the determination in tablets. Hence, the proposed methods could be adopted for quality control in pharmaceutical industries.

S. No. Reagent/s used Methodology λmax (nm) Linear range
(µgmL-1)
LOQ
(µg mL-1)
Reaction time Remarks Ref.
1. p-Chloroanilic acid Pink coloured C-T complex was measured. 519 10-200
(€=1.28×103
L mol-1cm-1)
- Instantaneous Less sensitive 14
2. 0.1 M NaOH LMT in NaOH was measured. 305 2-50 - - 21
3 a) BCG/Dichloromethane Ion-pair extraction 410 1.5-15 1.3 5 min Tedious extraction procedure involved and Less sensitive 22
b) Alcoholic KOH Ion-pair breaking 620 0.5-5.0 0.59 Instantaneous
4 a) BPB/CHCl3 Ion-pair extraction 420 2.5-25 µg mL-1 0.15 5 min Tedious extraction procedure involved  
b) Ethanolic H2SO4 Ion-pair breaking 420 50-400 ng mL-1 0.003 Instantaneous 23
  c) Ethanolic KOH 600 10-80 ng mL-1 0.0005
5 a) BCP/dichloromethane Ion-pair extraction 410 2.0-20.0 µg mL-1 0.8 5 min 24
b) Ethanol Ion-pair breaking 410 150-1500 ng mL-1 0.06 Instantaneous
c) Ethanolic KOH 600 50-600 ng mL-1 0.02
6 a) Bromocresol green/CHCl3 Ion-pair extraction NA 0.15-19.8 µg mL-1 NA NA 25
b) Bromocresol purple/CHCl3 NA 0.15-19.8 µg mL-1 NA NA
c) Chlorophenol red/CHCl3 NA 0.05–34.1 µg mL-1 NA NA
7 a) 0.1 M H2SO4 LMT in 0.1 M H2SO4 was measured. 225 0.5-5.0
(€=8.65×104
L mol-1cm-1)
0.02 - Very simple, and wide linear dynamic ranges. Present work.
b) Methanol LMT in methanol was measured. 225 1.25-12.5
(€=2.11×104
L mol-1cm-1)
0.09 -

Table 6: Performance characteristic of the existing spectrophotometric methods and the proposed methods.

Aknowledgement

Authors thank M/S. Cipla India Ltd, for gifting pure lamotrigine. Two of the authors (KBV and PJR) thanks the authorities of the University of Mysore, Mysore, for permission and facilities. One of the authors (NRP) thanks the J.S.S. institute for finding platform to pursue research work.

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