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, a nonpeptide tetrazole derivative, is an angiotensin receptor blocker (ARB) used mainly for the treatment of hypertension and diabetic nephropathy [65]. It is a specific competitive antagonist of AT 1 receptors with a much greater affinity (more than 8500-fold) for the AT 1 receptor than for the AT 2 receptor and has no agonist activity. The blocking of AT 1 receptor leads to multiple effects including vasodilation, a reduction in the secretion of vasopressin and reduction in the production and secretion of aldosterone. The resulting effect is a decrease in blood pressure. Unlike ACE inhibitors, irbesartan and other ARBs do not interfere with response to bradykinins and substance P, thus it does not cause dry cough [66]. Peak plasma levels are obtained approximately 1.5 to 2 hours after oral administration, and the plasma half-life ranges from 11 to 15 hours. is metabolized in part to the glucuronide conjugate, and along the parent compound it is cleared by renal elimination (20%) and biliary excretion (80%) [67]. The plasma clearance of irbesartan is unaffected by either renal or mild to moderate hepatic insufficiency. The oral dosage of irbesartan is 150 to 300 mg once daily. 5.1 Literature review of analytical methods Several analytical methods have been developed for the determination of irbesartan including liquid chromatography combined with UV detector [68, 69], diode array detector [70-72], fluorescence detector [73-77], electrospray ionization mass spectrometric detection [78] and tandem quadrupole mass spectrometer [79]. Capillary electrophoresis methods for analysis of several angiotensin receptor antagonists [80-83] have been reported. Determination of irbesartan alone or in combination in pharmaceutical dosage forms by spectrophotometry [84 86] has also reported. Stress degradation study and characterization of degradation product was not reported until we started to work on irbesartan, but recently Ravi Shah et.al reported the work using LC-NMR technique [87]. In this chapter the stress degradation study of irbesartan and its formulation, IRBEST tablet (Biochem) under different conditions has been studied and major degradation products has been characterized using different spectroscopic techniques. Stress study on formulation was performed in order to check the presence of any additional peak (due to drug-excipient interaction) other than those observed in stress study of API 64
5.2 Experimental work 5.2.1 Preparation of sample 5.2.1.1 Preparation of 0.01 N sodium hydroxide 0.4 g of sodium hydroxide (NaOH) was dissolved in sufficient quantity of water and diluted to make volume 100 ml to get 0.1 N sodium hydroxide solution. 1mL of this solution further diluted up to 10mL to get 0.01N NaOH solution. 5.2.1.2 Preparation of 1N and 2 N hydrochloric acid 8.5 ml of concentrated hydrochloric acid (HCl) was diluted to 100mL with water to get 1N HCl. Subsequently 17 ml of concentrated HCl was diluted to 100 ml with water to get 2 N HCl. 5.2.1.3 Preparation of 3% hydrogen peroxide 10 ml of hydrogen peroxide (30% w/v H 2 O 2 ) was diluted to 100 ml with water to get 3% hydrogen peroxide solution. 5.2.1.4 Preparation of 0.01 M potassium dihydrogen orthophosphate buffer (ph 2.7) 1.36 g of KH 2 PO 4 was dissolved in 1000 ml of water and ph was adjusted to 2.7 with orthophosphoric acid. 5.2.2 Forced degradation study was subjected to hydrolytic, oxidative, photolytic and thermal stress, as per ICH guidelines. Acid hydrolysis studies were carried out in 1N HCl at drug strength of 1mg ml -1. The solution was heated to 70 0 C for 7 days. Subsequently hydrolysis was also done using 2N HCl and heated at the same temperature and for 7days. Base hydrolysis was carried out at 50 0 C for 6 hrs while hydrolysis at neutral ph was done by suspending the drug in distilled water (ph 7) and heated at 70 0 C for 7 days. For oxidative study drug was dissolved in 3% H 2 O 2 at concentration of 1mg ml -1 and in 30% H 2 O 2 at concentration of 1 mg ml -1 and kept at room temperature for 7 days. 65
For thermal degradation in solid state drug powder was spread on a Petri dish to form a thin film about 1mm thickness. This petri dish was then kept in dry air oven at 70 0 C for 14 days. The sample was analyzed on 3 rd, 7 th, 10 th and 14 th day. Photolytic study was performed using photostability chamber. The drug was taken in petri-plate to make a thickness of 1mm and exposed at 40 0 C to 1.2 million lux hour of fluorescent light and 200 watt h m -2 of UV light. Dark control was run simultaneously. 5.2.3 High performance liquid chromatography (analytical) All the stressed samples were diluted with mobile phase to make concentration of 300 g ml -1 and injected on HPLC. A Phenomenex Gemini C 18 column with the dimension of 250mm 4.6mm internal diameter with 10 particle size was used for the separation. Flow rate was kept at 1.2 ml min -1 and the column eluent was monitored at a wavelength of 259 nm. The data was recorded using class VP Software. Mobile phase used was Potassium dihydrogen orthophosphate (0.01M) in water (pump A) and methanol (pump B), ph adjusted to 2.7 using ortho-phosphoric acid with gradient elution program 0.01/50, 1/50, 2/60, 4/70, 4.01/85, 12/85, 14/75, 16/65, 18/50 and 30/50 (time (min)/%b). 5.2.4 Semi-preparative HPLC For isolation of degradation products the analytical method was optimized for semi preparative HPLC considering parameters like loading capacity, flow rate, sample concentration and sample loop. Flow rate was increased to 4.75 ml min -1 and injection volume was increased to 100 L. mobile phase with same gradient program as in analytical method was used with the change in buffer. Instead of potassium dihydrogen orthophosphate formic acid (ph 2.7), a volatile buffer was used as it is easy to evaporate from collected fractions. The methanol from the collected fractions was removed by distilling under vacuum. The aqueous sample was then cooled to -80 0 C and then kept in lyophilizer to remove water. The lyophilized sample was then dissolved in dichloromethane and in that anhydrous sodium sulphate was added in order to remove any traces of water. It was then filtered and dichloromethane evaporated using rotavapor. The sample was again treated with hexane and ethyl acetate to remove any traces of impurity. The purity of final compound was determined by HPLC using PDA detector. 66
5.2.5 Characterization of degradation products 5.2.5.1 Mass spectroscopy Mass spectra were recorded using a quadrupole mass spectrometer (Perkin Elmer Sciex API- 165) equipped with an electro spray ion source. The scan was recorded over the range of m/z 100-800. The degradation products isolated by semi-preparative HPLC were dissolved in methanol and injected (2 L) through LC-MS (flow injection analysis) using acetonitrile: 0.1 % ammonium acetate (90:10 v/v) at 0.2mL min -1 flow rate in order to determine the molecular weight. The fragmentation pattern of the drug and degradation products was carried out using API-2000 LC/MS-MS triple quadrupole system with parameters: ionization source (+ve ESI), nebulizer pressure (nitrogen 20 psi), gas temperature (200 0 C), drying gas (nitrogen 30 psi) and capillary voltage (5500 V). The data was processed using Analyst 1.4 software. 5.2.5.2 FT IR Spectroscopy The IR spectra of drug and degradation products were recorded using Shimadzu IR Affinity 1 FT-IR instrument and IR solution software. Nearly 1mg of sample was thoroughly mixed with 100 mg of finely powdered KBr. The diffuse reflectance spectra (DRS) were recorded using DRS assembly which gives spectra similar to transmittance spectra. 5.2.5.3 NMR Spectroscopy 1 H and 13 C-NMR spectra of irbesartan and its degradation product were recorded on a Bruker Avance AV-300 NMR Spectrometer at frequencies of 300 MHz. The drug was dissolved in DMSO-d 6 while the degradation product was dissolved in CDCl 3. Tetramethylsilane was used as internal standard. 5.2.6 Validation of analytical method 5.2.6.1 Linearity and range A stock solution of the drug was prepared at strength of 1mg ml 1 in methanol. It was then diluted to solutions of concentration in the range of 20-100 g ml 1 in mixture of water and methanol (1:1). The solutions were injected in triplicate into the HPLC column, keeping the injection volume constant (20 L) 67
5.2.6.2 Specificity Specificity of the HPLC method was determined by spiking the degradation product and determining the resolution. Peak purity of drug and its degradation product was determined by PDA detector to rule out overlap of peak. 5.2.6.3 Precision Six injections of three different concentrations (20, 60 and 100 g ml 1 ) of formulation were injected and analyzed on the same day and the values of relative standard deviation (R.S.D.) were calculated to determine intra-day precision. These studies were also repeated on different days to determine inter-day precision. 5.2.6.4 Accuracy Accuracy was determined by spiking a mixture of stressed samples with three known concentrations of the drug i.e., 20, 60 and 100 g ml -1 and % recoveries of the added drug were calculated. 5.2.6.5 LOD and LOQ Limit of detection (LOD) and limit of quantification (LOQ): The LOD and LOQ for irbesartan was determined at a signal to-noise ratio of 3:1 and 10:1 respectively, by injecting a series of solutions with known concentrations. 5.3 Result and Discussion 5.3.1 Stress degradation study The chromatogram of irbesartan shows retention time of 11 minutes is as shown in Fig 5.1a. The drug was found to be very stable under acidic conditions since it did not degrade even in 2N HCl solution kept at 70 0 C for 7 days. One degradation product was observed at 10.7 min on heating the drug in 0.01N NaOH for 6 hr at 50 0 C indicating that is very susceptible to base hydrolysis (Fig 5.1b), while no additional peaks were generated on heating the drug in distilled water indicating the stability under neutral ph. There was insignificant degradation observed in oxidative stress using 3% as well as 30% H 2 O 2 at room temperature for 7 days indicating the drug was stable to oxidative stress. The drug was also found to be stable under thermal and photolytic stress conditions. Similar results were observed when identical stress 68
conditions were applied to the formulation indicating there was no drug-excipient interaction and thus the drug was found stable in presence of excipients. Fig 5.1 Chromatograms of a) irbesartan and its degradation under b) basic conditions 5.3.2 Isolation of degradation product The degradation product was separated and isolated.the purity of isolated degradation product was checked by HPLC using PDA detector and it was found to be 99.04 % with peak purity index of 0.9965 indicating peak is pure. 5.3.3 Characterization of degradation product To characterize the degradation product completely, different spectra of the degradation products were recorded and compared to that of the parent molecule to verify the changes occurred due to degradation. 69
5.3.3.1. Spectral data of The ESI mass spectrum of irbesartan showed a protonated molecular ion peak at m/z 429.3 confirming the molecular weight 428. The fragmentation pattern of parent ion 429.3 showed the fragment ions at m/z 385.9, 235.1, 207, 195.4, 192.1, 180.2 and 84 (Fig5.2). Fig. 5.2 Fragmentation pattern (MS 2 ) of irbesartan The FT-IR spectrum exhibited a characteristic stretching absorption band at 1732 cm -1 for the carbonyl group of amide functionality. The presence of this band at higher frequency was due to the ring stretching due to five member ring system. Another band at 1614cm -1 was due to C=N stretching vibrations (Fig 5.3). 70
Fig. 5.3 FT-IR spectrum of irbesartan 1 H and 13 C- NMR were recorded using DMSO-d 6 as a solvent. The spectra are as shown in Fig. 5.4. In 1 H-NMR the signal due to tetrazole NH proton was not detected may probably due to the tautomerism. 71
Fig. 5.4 1 H and 13 C-NMR spectrum of irbesartan 72
5.3.3.2. Structure elucidation of degradation product The molecular weight of degradation product by MS spectra was found 446 amu indicating the degradation product has a molecular mass of 18 amu higher than that of irbesartan, which may correspond to one oxygen and two hydrogen atoms. The mass spectrum with possible structure of degradation product is shown in Fig 5.5. Fig. 5.5 Mass spectra of DP-I with the possible structure The fragmentation (MS 2 ) of degradation product as in Fig. 5.6 shows major fragment ions at m/z 252.1, 235.1, 196.4, 168.3 and 84.2 which resembles the probable fragmentation pattern of structure as shown in Scheme 5.1. 73
Fig. 5.6 Fragmentation pattern (MS 2 ) of DP-I Scheme 5.1 Schematic representation of fragmentation pattern (MS 2 ) of DP-I 74
IR spectra of degradation product show a broad peak from 3400cm -1 to 2600cm -1 indicating presence of hydroxyl group. Presence two medium peaks at 3323 cm -1 and 3232 cm -1 indicating presence of two secondary amine groups. A shift of carbonyl peak from 1732cm -1 present in parent compound to 1658cm -1 in DP-I indicate there is a change in carbonyl functionality (Fig 5.7). Fig. 5.7 FT-IR spectrum of DP-I 1 H-NMR showed two additional proton signals, one at 7.82 ppm could be due to hydroxyl group of carboxylic acid and another signal for one proton at 1.96 ppm indicating presence of NH proton when compared with 1 H- NMR of parent compound. A signal at 6.66 ppm was due to tetrazol NH proton which was not observed in parent compound. In 13 C-NMR spectrum there was negligible shift in all signals except of carbonyl group which shifted to 174 ppm compared to 183 ppm in parent compound indicating change in carbonyl functionality. The chemical shift in upfield region was may be due to breaking of five member ring at 11 position in parent compound (Fig 5.8). 75
Fig 5.8 1 H and 13 C-NMR spectrum of DP-I 76
Fig 5.9 Structure of a) and b) DP-I Table 5.1 Comparative 1 H and 13 C- NMR assignments for a) in DMSO-d 6 and b) DP-I in CDCl 3 Position of Carbon (ppm) IRB (ppm) DP-I 13 C IRB 13 C DP-I 1 0.78 (t, 3H) 0.80 (t, 3H) 13.5 13.7 2 1.27 (m, 2H) 1.16-1.28 (m,2h) 21.4 22.2 3 1.48 (m, 2H) 1.16-1.28 (m,2h) 26.5 27.7 4 2.27 (t, 2H) 2.17 (t, 2H) 27.4 29.6 5 161.1 164.0 6 1.65(m, 2H) 1.50 (m, 2H) 36.7 36.7 7 1.80(m, 2H) 1.70 (m, 2H) 25.4 23.8 8 1.80(m, 2H) 1.70 (m, 2H) 25.4 23.8 9 1.80(m, 2H) 1.70 (m, 2H) 36.7 36.5 10 75.7 67.6 11 185.6 174 N-H - 1.96 (t, 1H) - 12 4.66(s, 2H) 4.27 (s, 2H) 42.2 43.2 13 138.3 138.2 14 7.074 (s, 1H) 6.94 (m, 1H) 126.2 127.6 15 7.074 (s, 1H) 7.05 (m, 1H) 129.2 129.1 16 140.9 141.1 17 7.074 (s, 1H) 7.05 (m, 1H) 129.2 129.1 18 7.074 (s, 1H) 6.94 (m, 1H) 126.2 127.6 19 131.0 131.2 20 7.69 (d, 1H) 7.55 (m,1h) 123.4 122.7 21 7.57 (m, 1H) 7.45 (m, 1H) 127.8 127.9 22 7.52 (m, 1H) 7.38 (m, 1H) 130.5 130.7 23 7.64(d, 1H) 7.55 (m,1h) 130.5 130.7 24 136.2 138.0 25 154.9 155.2 N-H a* 6.66 (s, 1H) O-H - 7.82 (t, 1H) *a- signal not observed 77
5.3.4 Plausible mechanism for formation In case of base hydrolysis of irbesartan, the hydroxyl group being a strong nucleophile, attacks on the electron deficient carbonyl functional group of the imidazolone ring. The result is the opening of the imidazolone ring with the formation of amidine type of intermediate formation (which is the DP-I) as described in scheme 5.2. Scheme 5.2 Plausible mechanism of formation of DP-I 5.3.5 Validation of LC method 5.3.5.1 Linearity The linearity of the method was established between concentrations of 20 g ml -1 to 100 g ml -1. The r 2 value was 0.999. Table 5.2 Linearity and range Conc. ( gml -1 ) 20 40 60 80 100 Area1 Area 2 Area 3 Average ± S.D ± R.S.D (%) 736543 1430175 2168452 3062925 3916572 729856 1421784 2136516 3095461 4024787 723874 1422936 2209645 3105386 4036183 730091 1424965 2171538 3087924 3992514 6337.77 4548.61 36662 22211.2 66014.1 0.86 0.31 1.68 0.71 1.65 5.3.5.2 Specificity Peak purity index of drug and its degradation product was more than 0.999 indicating the peaks are pure and there is no peak overlapping. The resolution between the parent compound and degradation product was more than 1.5 thus method is specific for the drug. 78
5.3.5.3 Precision The RSD for precision samples varied from 0.21 to 1.4% for all the experiments repeated for 3 consecutive days indicating the method is precise Table 5.3 Intra-day and Inter-day precision study Concentration ( gml -1 ) Intra Day Precision Measured Conc. ( gml -1 ) ± S.D., R.S.D (%) Inter Day Precision Measured Conc. ( gml -1 ) ± S.D., R.S.D (%) 20 19.163±0.0137,0.21 20. 616±0.1169,0.65 60 60.160±0.084,0.94 59.176±0.2464,1.40 100 99.661±0.0783,0.82 101.86±0.2797,1.03 5.3.5.4 Accuracy The percentage recovery of irbesartan from stress sample ranged from 99.3 to 100.1% indicating the accuracy of the method. Table 5.4 Recovery data for drug spiked into a mixture of stressed samples Spiked Conc. Calculated spiked Conc. ( g ml -1 ) ± Recovery (%) ( g ml -1 ) S.D., R.S.D (%) 20 20.02±0.0721,0.17 100.10 60 59.59±0.0143,0.08 99.31 100 99.81±0.0367,0.20 99.81 5.3.5.5 LOD and LOQ The limit of detection and limit of quantitation was found to be 0.7 g ml -1 and 1.8 g ml -1 respectively. 5.4 Summary The above studies revealed that irbesartan is unstable only under base hydrolytic condition. The degradation product was isolated and characterized as 1-(1-((2'-(1H-tetrazol-5- yl) biphenyl-4-yl) methylamino) pentylideneamino) cyclopentane carboxylic acid having molecular formula C 25 H 30 N 6 O 2. 79
The stability indicating assay method was developed and validated with linearity, accuracy precision LOD and LOQ. The plausible mechanism of formation of the degradation product was also established. 80