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Insight Pharmaceutical Sciences>>  Volume 8 Issue 1, 2018
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Stability-indicating UV-spectrophotometric Assay of Rifampicin
Nagaraju Swamy , Kanakapura Basavaiah and Penmatsa Vamsikrishna
 
ABSTRACT:
Background: Rifampicin (RIF), chemically known as 3-[[(4-methyl-1-piperazinyl)-imino]-methyl]-rifamycin SV is potentially hepatoxic and is an established first-line antituberculosis agent derived from rifamycin SV and its use in other serious infections, such as HIV is increasing. Objective: Two simple, sensitive, precise and economical UV-spectrophotometric methods were developed and validated for the determination of rifampicin (RIF) in bulk drug, capsule formulation and spiked human urine. Materials and Methods: The methods are based on the measurement of absorbance of RIF either in 0.1 M hydrochloric acid (HCl) at 263 nm (method A) or in 0.1 M orthophosphoric acid (H3PO4) at 259 nm (method B). Results: The methods were validated for linearity, accuracy and precision, limits of detection (LOD) and quantification (LOQ) and robustness and ruggedness as per the current ICH guidelines. Beer’s law was obeyed over concentration range of 1.5-30 μg mL–1 RIF in both methods with correlation coefficients of 0.9995 and 0.9997 for method A and method B, respectively. The corresponding molar absorptivity values were 2.49×104 and 2.71×104 L mol–1 cm–1. The utility of the methods was tested by their application to marketed formulation and the relative error and relative standard deviations were less than 2.1%. The validity and reliability of the proposed methods were further ascertained by the recovery studies via standard addition technique. In addition, forced degradation of RIF was conducted in accordance with the ICH guidelines. Acidic, basic, thermal, peroxide and photolytic stress conditions were used to assess the stability-indicating potency of the methods. Conclusion: Very slight degradation under peroxide-induced, slight degradation under acid-induced and substantial degradation under base-induced stress conditions were observed in both methods. No degradation was observed under other stress conditions. To enhance their usefulness, the methods were successfully applied to the determination of RIF in spiked human urine with satisfactory recovery.
 
    How to Cite:
Nagaraju Swamy, Kanakapura Basavaiah and Penmatsa Vamsikrishna , 2018. Stability-indicating UV-spectrophotometric Assay of Rifampicin. Insight Pharmaceutical Sciences, 8: 1-12
DOI: 10.5567/IPHARMA-IK.2018.1.12
 


INTRODUCTION
Rifampicin (RIF) (Fig. 1), chemically known as 3-[[(4-methyl-1-piperazinyl)-imino]-methyl]-rifamycin SV is potentially hepatoxic and is an established first-line antituberculosis agent derived from rifamycin SV and its use in other serious infections, such as HIV is increasing1. It is metabolized in the liver mainly by deacetylation and is excreted with its metabolites in bile. This requires careful monitoring of its serum concentration, when used by patients with liver disease to optimize the dose2.

Several methods have been reported for the determination of RIF in body fluids and include High Performance Liquid Chromatography (HPLC)3,4, cyclic and square wave voltammetry5, fluorimetry and microbiology6, nuclear magnetic resonance spectrometry7, chemometrics-aided kinetic spectrophotometry8 and visible spectrophotometry9-15. In combination with other antituberculosis drugs such as isoniazid, pyrazinamide and ethambutol, RIF in pharmaceutical dosage form has been assayed by HPLC16,17, HPTLC18,19, linear sweep and cyclic voltammetry20, cyclic and square wave voltammetry5, differential pulse polarography21,22 and chemiluminescence spectrometry23,24. The RIF, when present alone in capsules was quantitated by HPLC25, differential pulse polarography26,27, horseradish peroxidase-based amperometry28, chemiluminescence spectrometry29, nuclear magnetic resonance spectrometry7 and visible spectrophotometry9-15, 30-34.

The demand for fast and reliable measurement of RIF has necessiated the need for simple, easy to handle and inexpensive method. UV-spectrophotometry with its variants constitutes an evident alternative to the already existing methods. Multivariation calibration based on partial least square methods35-37 were presented for the simultaneous determination of RIF, isoniazid (INH) and pyrazinamide (PYR)35,36 and RIF, INH, PYR and ethambutol37. Benetton et al.38 described first derivative UV-spectrophotometric method for RIF and INH whereas the same method was employed by Rote and Sharma39 for the determination of RIF in combination with INH and PYR. A method based on the convolution of the double divisor ratio spectra was employed by Youssef and Maher40 for resolving and assaying a ternary mixture containing RIF, INH and PYR. Three methods41 based on the measurement of graphical absorbance ratio, derivative ratio and additivity of absorbances were developed for the simultaneous determination of RIF and INH. However, there is only one report on the application of direct UV-spectrophotometry9 for the assay of RIF in formulations in which absorbance of an aqueous solution was measured at 340 nm. However, this method is not stability-indicating.

Figure 1: Chemical structure of rifampicin

The need for a simple, sensitive, economical and faster analytical method in pharmaceutical quality control and clinical laboratories need not be overemphasized. Two UV-spectrophotometric methods which are stability-indicating were developed and validated for the determination of rifampicin in bulk drug, capsules and spiked human urine.

The stability-indicating assay is a method that is employed for the analysis of stability of samples in pharmaceutical industry. With the advent of ICH guidelines42,43, the requirement of establishment of Stability Indicating Assay Method (SIAM) has become more clearly mandated.

The guidelines explicitly require conduct of forced degradation studies under a variety of conditions like pH, light, oxidation, dry heat, etc. and separation of drug and degradation products. A review on the development of validated Stability Indicating Assay Methods (SIAMs) for drug substances and products is available44.

In this study, we developed and evaluated the performance of two simple, rapid and validated UV-spectrophotometric methods for RIF in pharmaceuticals and urine, which are stability-indicating. The methods are based on the measurement of the native absorbances of RIF in 0.1 M HCl (method A) and 0.1 M H3PO4 (method B). The methods were demonstrated to be sensitive, selective, accurate and precise, besides being robust and rugged.


MATERIALS AND METHODS
Materials and reagents: Pharmaceutical grade RIF (99.9% purity) was a gift from Lupin Limited, Tarapur, Maharashtra, India and was used as received. Capsules in two strengths R-Cin 300 and R-Cin 450 capsules (Lupin Limited, Chikaltana, Aurangabad, India) were purchased from local commercial stores. Solvents such as chloroform (99.4% alcohol stabilized and spectroscopic grade), sodium hydroxide, potassium hydrogen orthophosphate, hydrochloric acid, orthophosphoric acid and hydrogen peroxide were purchased from Merck, Mumbai, India. Double distilled water was used throughout the investigation.

Sodium hydroxide solution (NaOH, 5 M) was prepared by dissolving required amount of pellets in water. Hydrochloric acid (HCl, 5 M) was prepared by appropriate dilution of concentrated acid (Specific gravity 1.18) with water. This solution was diluted to 0.1 M and standardized45. Orthophosphoric acid (0.1 M) was prepared by appropriate dilution of concentrated acid (Specific gravity 1.57) with water. About 3% solution of H2O2 was prepared by diluting required volume of the commercially available 30% reagent with water. A phosphate buffer of pH 7.4 was prepared by mixing 50 mL of 0.1 M potassium hydrogen phosphate and 25 mL of 0.1 M NaOH, diluted to 100 mL with water and pH adjusted using pH meter. Human urine was collected from a healthy male aged about 30 years.

Preparation of standard drug solution: Two stock standard solutions of 100 μg mL–1 RIF both were prepared by dissolving 10 mg each of pure RIF separately in 0.1 M HCl (method A) and 0.1 M H3PO4 (method B) and diluted separately to 100 mL with respective solvents in calibrated flasks.

Procedures for calibration curve
Method A (Using 0.1 M HCl): Into a series of 10 mL calibrated flasks, aliquots of standard drug solution (0.15-3.0 mL of 100 μg mL–1) equivalent to 1.5-30 μg mL–1 RIF were accurately transferred and the volume was made up to the mark with 0.1 M HCl. The absorbance of each solution was then measured at 263 nm against 0.1 M HCl as the blank.

Method B (Using 0.1 M H3PO4): Aliquots (0.0, 0.15, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mL) of RIF standard solution (100 μg mL–1) were accurately measured into a series of 10 mL graduated flasks and the volume was made up to the mark with 0.1 M H3PO4 and the absorbance of each solution was measured at 259 nm against 0.1 M H3PO4 as reagent blank.

A calibration curve was prepared in both methods by plotting the absorbance versus concentration of drug. The concentration of the unknown was computed from the respective regression equation derived using the Beer’s law data in both cases.

Preparation of capsule extract and assay procedure: Contents of 20 capsules were pooled and pulverized. The amount of capsule powder equivalent to 10 mg RIF was quantitatively transferred into two separate 100 mL volumetric flasks. The content in each flask was shaken well with about 50 mL of 0.1 M HCl or 0.1 M H3PO4 separately for 20 min and the content was diluted to the mark with the respective solvent. It was filtered using Whatman No. 42 filter paper. First 10 mL portion of the filtrate was discarded and 2 mL portion of the subsequent portion was subjected to analysis following the general procedures described earlier.

Procedure for analysis of spiked human urine: To prepare spiked urine sample7, 10 mg of the pure RIF and 10 mL of urine sample were transferred into a separating funnel, mixed well till dissolution was complete. The solution was extracted with three 10 mL portion of chloroform and the organic layer was collected in a beaker after drying over anhydrous sodium sulphate. The solvent was evaporated to dryness. The resulting residue was reconstituted with either 0.1 M HCl and diluted to 100 mL or with 0.1 M H3PO4 before diluting to 100 mL with the same solvent. Then, the analysis was performed in replicates as described under general procedure for pure RIF by taking 2 mL of the resulting urine solution.

Forced degradation studies: A stock solution containing 10 mg of RIF either in 100 mL of 0.1 M HCl or in 100 mL of 0.1 M H3PO4 was prepared separately. This stock solution (100 μg mL–1) was used for forced degradation under acid, base and peroxide-induced stress conditions to provide an indication of the stability-indicating property of the methods. About 1.5 mL of this solution was accurately transferred to separate 10 mL volumetric flasks. Three milliliters each of 5 M HCl, 5 M NaOH or 3% H2O2 were added to the flasks separately and the flasks were heated for 3 h on a water bath maintained at 80°C. Then the solutions were cooled and neutralized by adding base or acid, the volume in each flask was brought to the mark with 0.1 M HCl or 0.1 M H3PO4 separately and absorbance measured at 263 nm in method A and 259 nm in method B. Solid state thermal degradation was carried out by exposing pure drug to dry heat at 105°C for 24 h. For photolytic degradation studies, pure drug in solid state was exposed to 1.2 million lux hours in a photo stability chamber for 24 h. The sample after exposure to heat and light was used to prepare 100 μg mL–1 solutions in 0.1 M HCl and 0.1 M H3PO4 separately and the absorbance measured at 263 nm versus 0.1 M HCl in method A and 259 nm versus 0.1 M H3PO4 in method B.

Linearity, limits of detection and quantification: Three series (analytical curves) of standard solutions of RIF were prepared separately by the dilution of the stock standard solution in 0.1 M HCl and 0.1 M H3PO4 and absorbances were measured, in triplicate, at 263 nm in method A and 259 nm in method B, respectively. The limits of detection (LOD) and quantification (LOQ) were calculated directly from the calibration plot using the formulae:

where, S is the standard deviation of blank absorbance values and b is the slope of the calibration plot43.

Within-day and day to day accuracy and precision: The within-day and day to day precisions of the proposed methods were evaluated by measuring the absorbance 7 times on the same day and on five different days using three different concentrations of RIF (10, 20 and 25 μg mL–1). From the absorbance values obtained, concentration was calculated and the results were expressed as percentage Relative Standard Deviation (RSD%). The accuracy was evaluated as percentage Relative Error (RE%) between the found and taken concentrations.

Accuracy: The accuracy and validity of the proposed methods were evaluated by performing recovery studies. Pre-analyzed tablet powder was spiked with pure RIF at three concentration levels (50, 100 and 150% of that in tablet powder) and the total was found by the proposed methods. The recovery (%) of the pure drug added was calculated as:

where, Ci is the total drug concentration measured after standard addition, Cs is drug concentration in the formulation sample and Ca is drug concentration added to the formulation42.

Robustness and ruggedness: Robustness of the proposed methods was determined by the analysis of samples and standard solutions (10, 20 and 25 μg mL–1) at different wavelengths. Ruggedness is a measure of reproducibility of test results under the variation in conditions normally expected from instrument to instrument and from analyst to analyst. Ruggedness of the proposed methods was determined by analysis of aliquots from homogeneous slot by three analysts using same operational and environmental conditions and by a single analyst with three different cuvettes.

Selectivity by placebo and synthetic mixture analyses: A placebo blank of the composition Barsoum et al.9: Urea (10 mg), sodium oxalate (15 mg), camphor (10 mg), glucose (10 mg), lactose (20 mg), sucrose (15 mg) and ascorbic acid (10 mg) was made and its solution was prepared as described ‘procedure for capsules’ and then subjected to analysis.

To assess the role of the inactive ingredients on the assay of RIF, a synthetic mixture was separately prepared by adding 10 mg of RIF to the 10 mg placebo. The drug was extracted and solution was prepared as described under the ‘procedure for capsules’. The solutions after appropriate dilution were analyzed following the recommended procedures.

RESULTS AND DISCUSSION

The aim of this study was to validate two simple, rapid and eco-friendly methods to assay RIF in formulations by UV-spectrophotometry. In method A 0.1 M HCl was used as the solvent and the absorbance was measured at 263 nm (Fig. 2a) where as in method B, 0.1 M H3PO4 was used as a solvent with the measurement being made at 259 nm analytical wavelength (Fig. 2b).

Validation protocol
Linearity: A linear relationship was found between the absorbances at 263 nm in method A and 259 nm in method B and the concentration of RIF in the range 1.5-30 μg mL–1 in both methods as shown in Fig. 3. The correlation coefficients (r) were 0.9995 and 0.9997 indicating good linearity. The representative linear equations were y = 0.0303x+0.0008 and y = 0.0316x+0.0046 calculated by the respective least squares method, for method A and method B, respectively. Optical characteristics such as Beer’s law limits, molar absorptivity and Sandell sensitivity values were calculated. The limits of detection (LOD) and quantification (LOQ) were also calculated and all these data are presented in Table 1. The uncertainties with the y-axis (Sy), intercept (Sa) and slope (Sb) were also calculated for both methods. These results are presented in Table 1.

Precision and accuracy: The results of within-day and day to day analysis of the sample are given in Table 2. As evident, RSD% values of the data obtained were all below 3% (i.e., in the range of 1.67-2.72 and 1.58-2.29% for within-day and day to day, respectively). The RSD% values indicated that the proposed methods are sufficiently precise.

Figure 2(a-b): UV absorption spectra of (a) RIF (15 μg mL–1) in 0.1 M HCl and (b) RIF (15 μg mL–1) in 0.1 M H3PO4

Table 1: Sensitivity and regression parameters

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

As shown from the data presented in Table 2, the relative error between the taken and found concentrations of RIF is <2.05% indicating that the proposed methods are quite accurate for the assay of RIF.

Figure 3(a-b): Calibration plot for (a) Method A and (b) Method B

Table 3: Results of robustness and ruggedness expressed as intermediate precision

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

Robustness and ruggedness: In addition, the reliability of the proposed methods was also evaluated by means of robustness test. The absorbances of standard and sample solutions were determined at the UV wavelength used in this study λmax±1 nm. No significant difference was observed in the results found. Intermediate precision values (RSD%) were in the range 0.54-2.63% indicating acceptable robustness. Percent RSD values obtained as a part of the ruggedness study by recording the absorbance values using three cuvettes by a single analyst and by three analysts with a single cuvette are also presented in Table 3.

Application to capsule analysis: Commercial RIF capsules were analyzed using the proposed methods and also by a reference method38. Capsule extract equivalent to 100 μg mL–1 RIF was prepared in methanol and 5 mL of this extract was diluted to 10 mL with phosphate buffer of pH 7.4 and absorbance measured at 475 nm vs buffer. The results obtained were compared statistically by the Student’s t-test and the variance-ratio F-test. The calculated t- and F-values did not exceed the tabulated values of 2.77 and 6.39 at the 95% confidence level and for four degrees of freedom (Table 4), indicating close similarity between the proposed method and the reference method with respect to accuracy and precision.

Recovery study: To further ascertain the accuracy and reliability of the proposed methods, recovery experiments were performed via standard-addition technique. Pre-analyzed capsule powder was spiked with pure RIF at three different levels and the total was found by the proposed methods. Each determination was repeated 3 times. The percent recovery of pure RIF added was within the permissible limits indicating the absence of inactive ingredients in the assay. These results are as illustrated in Table 5.

Application to spiked human urine: The proposed methods were applied to human urine by taking 20 μg mL–1 extracted urine solution in replicates (n = 5). The recoveries obtained is 91.74% with standard deviation 0.91 for method A and 92.16 with 0.57 for method B indicated the accessibility of the methods and the results are given in Table 6.

Selectivity: The absorbance of the placebo solution in each case was almost equal to the absorbance of the blank which revealed no interference. The absorbance resulting from 20 μg mL–1 (in both methods) was nearly the same as those obtained for pure RIF solutions of identical concentrations. This unequivocally demonstrated the non-interference of the inactive ingredients in the assay of RIF.

Stability-indicating property: The RIF was subjected to acid, base and hydrogen peroxide induced degradation in solution state and photo and thermal degradation in solid state. The study was performed by measuring the absorbance of RIF solution only after subjecting to forced degradation. From the response, percentage recovery of RIF was calculated in each case and is presented in Table 7. Degradation study showed that very slight degradation was observed under peroxide-slight degradation under acid and substantial degradation under-base induced degradation in both methods. No degradation was observed under other stress conditions (Fig. 4-8).

Table 5: Results of recovery experiment via standard-addition procedure

Table 6: RIF determination in spiked urine sample (n = 5)

Table 7: Results of forced degradation studies

Figure 4(a-b): UV absorption spectra of thermal degraded product (a) RIF (15 μg mL–1) at 259 nm in 0.1 M HCl and (b) RIF (15 μg mL–1) in 0.1 M H3PO4

Figure 5(a-b): UV absorption spectra of photolytic degraded product (a) RIF (15 μg mL–1) at 259 nm in 0.1 M HCl and (b) RIF (15 μg mL–1) in 0.1 M H3PO4

Figure 6 (a-b): UV absorption spectra of acid degraded product (a) RIF (15 μg mL–1) at 259 nm in 0.1 M HCl and (b) RIF (15 μg mL–1) in 0.1 M H3PO4

Figure 7(a-b): UV absorption spectra of base degraded product (a) RIF (15 μg mL–1) at 259 nm in 0.1 M HCl and (b) RIF (15 μg mL–1) in 0.1 M H3PO4

Figure 8(a-b): UV absorption spectra of oxidative degraded product (a) RIF (15 μg mL–1) at 259 nm in 0.1 M HCl and (b) RIF (15 μg mL–1) in 0.1 M H3PO4


CONCLUSION
Two simple, rapid and sensitive methods are reported for the determination of rifampicin in the bulk form, capsules and spiked-human urine. This is the first study on the stability-indicating methods for rifampicin. The methods presented here are highly sensitive and rapid and require no organic solvents or any additional reagents. Further the methods are free from any tedious procedural or extraction steps. The instrument employed is cheap, easy to handle and no expertise personnel is required. They can be considered to be a promising alternative to HPLC. The proposed methods show clear advantages, such as short analysis time and no pretreatment or time-consuming extraction step (except for urine) were required prior to analysis. Moreover, because of its low limits of detection and quantification, the methods could be applied in clinical laboratories and pharmacokinetic studies.


ACKNOWLEDGMENTS
The authors acknowledge the receipt of pure rifampicin as gift from Lupin Limited, Tarapur, Maharashtra, India. Professor K. Basavaiah thanks UGC, New Delhi for the award of UGC-BSR faculty fellowship. Two of the authors (NS and PV), thank the authorities of the University of Mysuru, for providing research facilities.


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