75 CHAPTER 3 DEVELOPMENT AND APPLICATION OF STABILITY-INDICATING HPLC METHOD FOR THE DETERMINATION OF NEVIRAPINE AND ITS IMPURITIES IN COMBINATION DRUG PRODUCT 3.1 INTRODUCTION OF DOSAGE FORM AND LITERATURE REVIEW Combination therapy has proven to be one of the most effective approaches to treat HIV infection. Nevirapine is a non-nucleoside reverse transcriptase inhibitor (NNRTI), which acts against human immuno-deficiency virus type 1 (HIV-1). The drug is currently marketed for the treatment of adults with HIV-1 infection. Nevirapine is recommended for treating HIV infections in combination with other reverse transcriptase inhibitors like stavudine, zidovudine and lamivudine (Sweetman 2009). The analytical method has been reported for the individual nevirapine in EP (2008) and USP (2008). Kaul et al (2004) reported the HPTLC method for the determination of nevirapine in the pharmaceutical dosage form. Ananthan Kumar et al (2010), Namita et al (2006) and Samee et al (2007) published method for the estimation of nevirapine with other antiviral drugs. Some methods were published in biological fluid samples (Ghosh et al 2011, Omary et al 2010, Venkata Kumar et al 2010 and Vogel et al 2010). Castro et al (2011) estimated nevirapine through stripping voltammetry. Sreevidya and Narayana (2010) estimated this drug through spectrophotometry using tetrathiocyanatocobalt (II) ion as a reagent.
76 3.1.1 Target of the Work There is no stability-indicating method reported yet for the determination of nevirapine and its impurities in the combination drug product. To meet the requirements of pharmaceutical quality control analysis, a simple practical method is required for this combination drug product. It is very important to develop a simple, precise and reliable RP-HPLC method for the simultaneous estimation of the above mentioned components. Therefore, the focus of the study was to develop a stability-indicating RP-HPLC method for the combination drug product by degrading the drugs together under various stress conditions according to ICH guidelines. 3.2 EXPERIMENTAL 3.2.1 Materials and Reagents Pharmaceutical grade of nevirapine (chemically: 1,1-Cyclopropyl-4- methyl-5,1,1-dihydro-6h-dipyrido[3,2-b:2,3 -e][1,4]diazepin-6-one), lamivudine (chemically: 4-Amino-1-[(2R,5S)-2-(hydroxymethyl)-1,3-oxathiolan-5-yl] pyrimidine-2(1h)-one) and zidovudine (chemically: 1-(3-azido-2,3-dideoxy-β-Derythro-pentofuranosyl)-5-methylpyrimidine-2,4(1H,3H)-dione) were obtained as gift samples from Pharma Lab (Baddi, India). Nevirapine related compound A (chemically: 1,1-ethyl-4-methyl-5,1,1 dihydro-6h-dipyridol [3,2-b:2,3 - e][1,4]diazi pine-6-one), nevirapine related compound B (chemically: 4-methyl- 5,1,1dihydro-6H-dipyridol[3,2-b:2,3 -e][1,4]diazipine-6-one), zidovudine related compound B (chemically: 3 -chloro-3 -deoxythymidine) and lamivudine resolution mixture containing lamivudine and diastereomer impurities were purchased from LGC Standards (Mumbai, India). Salicylic acid (chemically: 2- Hydroxybenzoicacid) was purchased from Aldrich (Bangalore, India). Thymine impurity (chemically: 5-methylpyrimidine-2,4(1H,3H)-dione) was obtained from
77 Across Organics (Bangalore, India). Chemical structures are shown in Figures 3.1 to 3.9. The combination drug product containing lamivudine, zidovudine and nevirapine was purchased from nearby pharmacies. Each tablet contains 150 mg of lamivudine, 300 mg of zidovudine and 200 mg of nevirapine. HPLC grade acetonitrile was purchased from Merck (India). Buffer materials and all other chemicals were of analytical reagent grade. High purity water was manufactured using a Millipore Milli-Q plus purification system (Bedford, MA, USA). Figure 3.1 Chemical structure of nevirapine (MF: C 15 H 14 N 4 O, MW: 266) Figure 3.2 Chemical structure of lamivudine (MF: C 8 H 11 N 3 O 3 S, MW: 229)
78 Figure 3.3 Chemical structure of zidovudine (MF: C 10 H 13 N 5 O 4, MW: 267) Figure 3.4 Chemical structure of nevirapine related compound A (MF: C 13 H 12 N 4 O, MW: 240) Figure 3.5 Chemical structure of nevirapine related compound B (MF: C 12 H 10 N 4 O, MW: 226)
79 Figure 3.6 Chemical structure of lamivudine diastereomer (MF: C 8 H 11 N 3 O 3 S, MW: 229) Figure 3.7 Chemical structure of salicylic acid (MF: C 7 H 6 O 3, MW: 138) Figure 3.8 Chemical structure of thymine (MF: C 5 H 6 N 2 O 2, MW: 126)
80 Figure 3.9 Chemical structure of zidovudine related compound B (MF: C 10 H 13 Cl N 2 O 4, MW: 261) 3.2.2 Instrumentation The Waters HPLC system consisting of 2695 binary pump plus auto sampler, a 2996 photo diode array and a 2487 UV detector (Waters Corporation, Milford, USA) was used for the development and validation. 3.2.3 Preparation of Standard Solution A standard solution of nevirapine at the target concentration of 240 µg/ml, chosen for this study, was prepared by transferring 24 mg of standard solution into a 100 ml volumetric flask containing about 60 ml of diluents. The solution was sonicated for 30 min or until the solid completely dissolved keeping the water in the sonicator at an ambient temperature. Once dissolved, the volumetric flask was filled to mark with diluents. This standard solution was used for the assay determination of nevirapine. A stock solution of related compound A and related compound B at 0.01 mg/ml was prepared in a 100 ml volumetric flask, filled to volume with diluent and thoroughly mixed. Next, a diluted 2.5 ml of the resulting solution was prepared in a 100 ml volumetric flask, filled to volume with diluent and thoroughly mixed. This solution was used for the determination of related
81 compound A and related compound B. This solution corresponding to 0.1 % of the target concentration of nevirapine (240 µg/ml). 3.2.4 Preparation of Sample Solution Twenty tablets were weighed and crushed to fine powder. The powder, equivalent to 24 mg of nevirapine, was weighed in a 100 ml volumetric flask and around 60 ml of diluent was added. After sonication for 30 min, the solution was cooled and made up to the mark with diluents. The solution was centrifuged and the supernatant was used for the analysis. This sample preparation is used for the estimation of nevirapine, related compound A and related compound B. 3.3 RESULTS AND DISCUSSION 3.3.1 Optimization of Chromatographic Method The main objective of the chromatographic method is to separate nevirapine from its related compound A, related compound B and other sample matrices. By increasing the acetonitrile concentration, sample matrices like lamivudine, zidovudine and their impurities (Salicylic acid, lamivudine diasteriomer impurity, thymine and zidovudine related compound B) were eluted in the column void. The optimized chromatographic method is shown in Table 3.1. The developed LC method was found to be specific and selective for nevirapine and its impurities (Related compound A and related compound B in combination drug product). The peak shape of the nevirapine, related compound A and related compound B was found to be symmetrical. The representative chromatogram is shown in Figure 3.10.
82 Table 3.1 Optimized chromatographic method Buffer 0.05 M mono basic ammonium phosphate buffer (ph 4.5 adjusted with dilute sodium hydroxide solution) Mobile phase Buffer:Acetonitrile (7:3, v/v) Diluent Mixture of mobile phase and acetonitrile (9:1, v/v) Column Supelcosil ABZ, 150 mm x 4.6 mm, 5 micron Column oven temperature 30 C Detection wavelength 220 nm Injection volume 50 µl Flow rate 1.2 ml/min Figure 3.10 Typical HPLC overlay chromatograms of normal and impurity spiked samples (NV-nevirapine, RCB-related compound B and RCA-related compound A) 3.3.2 Method Validation The developed HPLC method was validated according to ICH and FDA guidelines in terms of precision, ruggedness, linearity, specificity, selectivity, robustness, LOD, LOQ and accuracy.
83 3.3.2.1 System Suitability Six injections of standard solution were used for system suitability check. System suitability was analyzed in terms of USP tailing factor (< 2.0) and USP theoretical plate counts (> 5000) of the components. The resolution between close eluting impurity of related compound B and nevirapine should not be less than 2.0. System suitability results are shown in Table 3.2. Table 3.2 System suitability results Parameter Retention time Nevirapine 8.77 Related compound B 6.26 Related compound A 13.99 Relative retention time - 0.71 1.59 USP resolution 2.82 - - USP tailing factor 1.02 0.92 1.12 USP theoretical plates 6921 7570 8575 3.3.2.2 Specificity and Selectivity Specificity is the ability of the method to measure the analyte response in the presence of its potential impurities and other sample matrices. Stress studies were performed in the combination drug product to provide stabilityindicating property and specificity for the proposed method. Intentional degradation was attempted to a stress condition of heat (80 C), humidity (85 %), light (254 nm), acid (0.1 N HCl), base (0.1 N NaOH) and peroxide (3 % H 2 O 2 ) to evaluate the proposed method s ability to separate nevirapine from its degradation impurities and sample matrices. For heat, light and humidity studies, the study period was 2 days, whereas for the acid, base and oxidation, it was 6 hours. Peak purity test was carried out for nevirapine, related compound A and related
84 compound B peak by using a PDA detector in stress samples. Assay studies were carried out for stress samples against the qualified nevirapine standard. Assay was also calculated for nevirapine samples by spiking the nevirapine related compound A and related compound B at the specification level (0.1 %). To prove the proposed method s selectivity, all individual compounds of nevirapine, nevirapine related compound A, nevirapine related compound B and other compounds, lamivudine and its impurities (Salicylic acid and diasteriomer impurity), zidovudine and its impurities (Thymine and zidovudine related compound B) were injected. The impurity details were obtained from USP (2008). Degradation was not observed in the sample when subjected to stress conditions like humidity, light and heat. Minor degradation was observed in base hydrolysis. One unknown degradation impurity was formed at the retention time of 6.8 min, with more than 1.3 USP resolution from close-eluting impurity of related compound B. Major degradation was observed in acid hydrolysis (Unknown degradation impurity RTs are 4.87 min, 6.76 min, 21.55 min, and 31.16 min) and peroxide oxidation (Unknown degradation impurity RTs are 4.86 min, 6.75 min, and 3.72 min). The forced degradation chromatograms are shown in Figure 3.11. Peak purity test results confirmed that nevirapine, related compound A and related compound B peak are homogenous and pure in all the analysed stress samples. More than 1.2 resolutions were found in all degradation impurities. The assay of nevirapine is unaffected in the presence of related compound A, related compound B and other sample matrices, which confirms the stability-indicating power of the method. The specificity of the developed LC method for nevirapine was revealed in the presence of its impurities (Related compound A and related compound B). The summary of forced degradation studies are given in Table 3.3.
85 Table 3.3 Forced degradation study results Condition Normal sample Acid Hydrolysis (0.1 N HCl) Base Hydrolysis (0.1 N NaOH) Oxidation (3 % H 2 O 2 ) Thermal (80 C) UV (254 nm) Humidity (85 %) % Assay of Nevirapine 99.8 94.2 98.9 95.4 99.8 99.8 99.1 % Mass Balance (% Assay + % Impurity) 99.9 99.3 99.2 99.3 99.9 99.9 99.2
86 Figure 3.11 Typical HPLC chromatograms of tablet under stress conditions: Normal, acid, base and peroxide. (NV- nevirapine, RCB- related compound B and RCA- related compound A)
87 The individual injections of nevirapine, related compound A, related compound B, lamivudine, zidovudine, thymine, zidovudine related compound B, lamivudine diastereomer and salicylic acid further prove the method s selectivity. Diluent was injected as a blank and no interference was found at the peak RT of nevirapine, related compound B and related compound A, which prove the developed method s specificity. The overlay chromatograms of individual injection and blank are shown in Figure 3.12. Figure 3.12 Typical HPLC overlay chromatograms of blank (A), nevirapine (B), related compound A (C), related compound B (D), lamivudine (E), lamivudine resolution mixture containing lamivudine and its diasteriomer impurity (F), salicylic acid (G), thymine (H), zidovudine related compound B (I) and zidovudine (J)
88 3.3.2.3 Precision The precision of the assay method was evaluated by carrying out six independent assays of the nevirapine test sample against a qualified standard and calculating the % RSD of the assay. The precision of the related substances was checked by performing six individual preparations of nevirapine spiked with 0.1 % of nevirapine related compound A and related compound B with respect to the nevirapine analyte concentration (240 µg/ ml). The % RSD for the nevirapine related compound A and related compound B was calculated. The intermediate precision of the method was also performed using different analysts, different lot columns and different instruments in the same laboratory. The % RSD of assay of nevirapine during the method precision and intermediate precision study was within 0.5 % and the % RSD for the area of related compound A and related compound B in related substances method precision and intermediate precision study was within 5 %, confirming good precision of the method. Precision data results are shown in Tables 3.4 and 3.5. Table 3.4 Method precision results Injection Nevirapine (%) 1 100.1 2 100.1 3 100.2 4 100.1 5 99.2 6 100.1 Mean 99.97 SD 0.38 % RSD 0.38 Related compound A (%) 0.12 0.12 0.01 4.08 Related compound B (%) 0.14 0.14 0.01 3.87
89 Table 3.5 Intermediate precision results Injection Nevirapine (%) 1 99.3 2 99.1 3 99.5 4 99.8 5 99.0 6 99.1 Mean 99.3 SD 0.30 % RSD 0.31 Related compound A (%) 0.12 0.12 0.12 0.01 4.48 Related compound B (%) 0.14 0.14 0.14 0.14 0.01 4.06 3.3.2.4 Limit of Detection (LOD) and Limit of Quantification (LOQ) LOD and LOQ for related compound A and related compound B were determined by the slope method, where a series of dilute solution with a known concentration was injected. Limit of detection was measured as the lowest amount of the analyte, where a significant response could be detected which is different from that of a blank. Limit of detection and limit of quantification were approved by calculations based on the standard deviation of the response (σ) and slope (S) of the calibration curve at the levels of approaching the limits according to equation LOD = 3.3 (σ/s) and LOQ = 10 (σ/s). Precision study was also carried out at the LOQ level by injecting six individual preparations of related compound A and related compound B and calculating the % RSD of the area. LODs for related compound A and related compound B were 0.008 % and 0.003 % respectively. Limit of quantification (LOQ) was measured as the lowest amount of analyte that could be reproducibly quantified above the baseline
90 noise, for which six injections resulted in an RSD 5 %. A practical LOQ giving a good precision was 0.02 % for both the related compound A and related compound B. The results of LOQ level precision are shown in Table 3.6. Table 3.6 LOQ level precision for impurities Peak Area Injection Related compound A Related compound B 1 2 3 4 5 6 Mean SD % RSD 8511 8423 8911 8255 8476 8423 8499.8 219.8 2.6 10200 9822 9911 9652 10111 9855 9925.2 200.2 2.0 3.3.2.5 Linearity Linearity test solution for the nevirapine assay method was prepared at five concentration levels from 50 % to 150 % of assay analyte concentration, i.e., 120, 180, 240, 300 and 360 µg/ml. The peak area versus concentration data was treated by least squares linear regression analysis. Linearity test solution for the related substances was prepared by diluting the stock solution to the required concentrations. The solution was prepared at six concentration levels from LOQ to 300 % (0.72 µg/ml) of the specification level (LOQ, 0.05 %, 0.1 %, 0.15 %, 0.2 % and 0.3 %).
91 The linearity calibration plot for the assay method was obtained over the calibration ranges tested, i.e., 120 to 360 µg/ml, and correlation coefficient obtained was greater than 0.999. The results show that an excellent correlation existed between the peak area and the concentration of the analyte. Linear calibration plot for the related substance method was obtained over the calibration range tested, i.e., LOQ (0.02 %) to 0.3 % for related compound A and related compound B. The correlation coefficient obtained was greater than 0.999. The results show that an excellent correlation existed between the peak area and the concentration of related compound A and related compound B. This linearity was represented by a linear regression equation. Linearity graphs are shown in Figures 3.13 to 3.15. Y NV = 164488x + 24200 Y RCA = 172756x - 555.6 (r=0.9996) (r=0.9996) Y RCB = 201923x + 500.95 (r=0.9999) Nevirapine Linearity Graph Peak Area 70000000 60000000 50000000 40000000 30000000 20000000 10000000 0 y = 164488x + 24200 R 2 = 0.9996 0 50 100 150 200 250 300 350 400 Concentration (ppm) Figure 3.13 Linearity graph for nevirapine
92 Related Compound A Linearity Graph Peak Area 140000 120000 100000 80000 60000 40000 20000 0 y = 172756x - 555.6 R 2 = 0.9996 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Concentration (ppm) Figure 3.14 Linearity graph for related compound A Related Compound B Linearity Graph Peak Area 160000 140000 120000 100000 80000 60000 40000 20000 0 y = 201923x + 500.95 R 2 = 0.9999 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Concentration (ppm) Figure 3.15 Linearity graph for related compound B
93 3.3.2.6 Accuracy The recovery experiments were carried out by spiking the already analyzed samples of the tablet with five different concentrations of standard nevirapine, i.e., 110 %, 120 %, 130 %, 140 % and 150 %. The rcentages of recoveries were calculated. The accuracy of the related substance was carried out in a triplicate at LOQ, 0.05 %, 0.1 %, 0.15 %, 0.2 % and 0.3 % of the nevirapine analyte concentration (240 µg/ml). The percentage recoveries for impurities were calculated. The percentages recovery of nevirapine in tablet sample ranged between 98.2 % and 101.5 %. The percentage recoveries of impurities in tablet samples were varied from 96.2 % to 104.3 %. Recovery results are shown in Table 3.7.
94 Table 3.7 Accuracy results Compound Level (%) 110 Amount added (µg/ml) 264 Recovery (%) 99.1 % RSD (n = 3) 0.2 120 288 98.2 0.5 Nevirapine 130 312 98.5 0.8 140 336 100.1 1.1 150 360 101.5 1.3 0.02 0.05 97.2 4.8 0.05 0.12 98.2 3.2 Related 0.10 0.24 97.5 4.1 compound A 0.15 0.36 100.1 3.9 0.20 0.48 102.3 2.8 0.30 0.72 96.2 3.1 0.02 0.05 102.4 3.9 0.05 0.12 100.5 3.5 Related 0.10 0.24 96.8 3.3 compound B 0.15 0.36 103.3 4.4 0.20 0.48 104.3 4.2 0.30 0.72 99.2 3.9
95 3.3.2.7 Robustness By introducing small changes in chromatographic parameters, the effects of the results were examined. The flow rate of the mobile phase was 1.2 ml/min. To study the effect of flow rate, flow was changed by 0.1 unit (From 1.1 to 1.3 ml/min) and the effect of the column temperature was studied at 28 C and 32 C instead of 30 C. The effect of solvent concentration was studied at ± 5 % from the nominal concentration. The ph of the buffer solution was studied at 4.3 and 4.7 instead of 4.5. In all deliberately varied chromatographic conditions, i.e., flow rate, column temperature, organic solvent and ph, the resolution between the critical pairs of nevirapine and related compound B was greater than 2. The assay of nevirapine was obtained well within the limit, i.e., between 98 % and 102 %, illustrating the robustness of the method. The robustness results are shown in Table 3.8.
96 Table 3.8 Robustness results Variations Flow rate 1.2 ml/min (original) 1.1 ml/ min 1.3 ml/min ph 4.5 (original) 4.3 4.7 Column temperature 30 C (original) 28 C 32 C Organic composition Buffer:Acetonitrile (70:30, v/v) (original) Buffer : Acetonitrile (75:25, v/v) Buffer : Acetonitrile (65:35, v/v) Resolution between nevirapine and related compound B 2.84 3.10 2.65 2.84 2.81 2.87 2.84 2.79 2.81 2.84 3.59 2.22 Assay of nevirapine (%) 99.30 98.25 99.51 99.30 100.10 100.52 99.30 99.11 98.75 99.30 99.82 100.20
97 3.3.2.8 Application of the Developed Method The method was applied for the assay of nevirapine and its impurities in market sample tablets (Tablet contains 150 mg lamivudine, 300 mg zidovudine and 200 mg nevirapine manufactured by Hetero). Nevirapine content was 99.3 %. The percentage of related compound A and related compound B was 0.03 % and 0.04 % respectively. 3.3.2.9 Conclusion RP-HPLC method was developed and validated for the determination of nevirapine and its impurities. The developed method is precise, accurate, linear, selective and specific. The method was validated showing satisfactory data for the method validation parameters tested. The developed method is stabilityindicating and can be conveniently used by the quality control department to determine the related substances and assay of nevirapine in regular samples and stability samples.