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Acta Pharm. 59 (2009) 121132 10.2478/v10007-009-0001-3 Original research paper Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation VARSHA POKHARKAR* ABHISHEK KHANNA VINOD VENKATPURWAR SHEETAL DHAR LEENATA MANDPE Department of Pharmaceutics Bharati Vidyapeeth University Poona College of Pharmacy Erandwane, Pune-411038 Maharashtra, India The purpose of this study was to improve the solubility and dissolution rate of carvedilol by forming a ternary complex with b-cyclodextrin and citric acid and to formulate its mouth-dissolving tablets. The rationale for preparing mouth-dissolving tablet of carvedilol was to make the drug available in a soluble form in the mouth, which would facilitate its absorption from the buccal cavity. This would help to overcome its first-pass metabolism and thereby improve bioavailability. Phase solubility studies revealed the ability of b-cyclodextrin and citric acid to complex with carvedilol and significantly increase its solubility. Ternary complexation of carvedilol was carried out with b-cyclodextrin and citric acid by physical mixing, kneading and spray drying methods and the prepared complexes were characterized by Fourier transform infra red spectroscopy, differential scanning calorimetry, powder X-ray diffractometry, scanning electron microscopy and complexation efficiency. The complex obtained by the spray drying method resulted in highest complexation efficiency and a 110-fold increase in the solubility of carvedilol. The mouth-dissolving tablets formulated using the spray dried complex with suitable excipients showed 100 % dissolution within five minutes. Accelerated stability studies of mouth-dissolving tablets carried out as per ICH guidelines revealed that the tablets were stable. Keywords: carvedilol, cyclodextrin, citric acid, ternary complex, solubility, spray drying, mouth-dissolving tablet Accepted March 11, 2009 Carvedilol is a non-selective beta-blocker indicated in the treatment of mild to moderate congestive heart failure. It is a basic, lipophilic compound. Being categorized as Class II compound as per the BCS classification system, it possesses very poor oral bioavailability and shows significant first pass metabolism (1). Thus, a strategy to improve bioavailability should aim at improving its aqueous solubility and overcoming first pass * Correspondence; e-mail: email@example.com V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. metabolism. Innovative drug delivery systems known as »melt in mouth« or »mouth dissolve tablets« are novel types of tablets that disintegrate/disperse/dissolve in saliva. Their characteristic advantage, such as administration without water anywhere anytime, leads to their suitability for geriatric and pediatric patients. They are also most suitable for drugs that undergo extensive fist pass metabolism. The benefits, in terms of patient compliance, rapid onset of action as the drug goes directly into systemic circulation and good stability, make these tablets popular as a dosage form of choice on the current market. However, a major challenge is to develop mouth-dissolving tablets of poorly soluble drugs (2). Of the various approaches to improve drug solubility, complexation with cyclodextrin is being widely explored. Cyclodextrins are powerful carriers for improving the therapeutic efficacy of drugs with poor aqueous solubility through inclusion complexes. However, exploitation of this cyclodextrin property is hindered by the relatively low water solubility and high molecular mass of natural cyclodextrin (3, 4). Among the strategies proposed for this aim, addition of suitable auxiliary substances can be a valuable approach to increase the cyclodextrin solubilization capacity. It has been reported that certain low molecular mass acids or hydroxyl acids e.g., citric acid, tartaric acid, can strongly enhance the cyclodextrin solubilizing power towards basic drugs as a result of the combined effect of salt formation and inclusion complexation (5, 6). The purpose of this study was to investigate a dual approach by forming an inclusion complex along with an appropriate salt form of carvedilol in order to improve its aqueous solubility and dissolution rate. EXPERIMENTAL Materials Carvedilol was a generous gift from M/s Symed Labs (India). b-cyclodextrin was purchased from M/s ISP Ltd. (Hong Kong). Citric acid, potassium dihydrogen phosphate, sodium hydroxide, calcium chloride were supplied by M/s Merck Ltd. (India). All other reagents and solvents used were of analytical grade. Analysis of carvedilol using a UV-visible spectrophotometer First, 100 mg mL1 stock solution of carvedilol was prepared in methanol and further dilutions were made in distilled water ranging from 530 mg mL1. The calibration curve was obtained by recording the absorbance on a UV spectrophotometer (Jasco V350, Japan) at 285 nm (7). Linearity in the calibration curve was observed with regression R2 = 0.9995. Phase solubility studies with b-cyclodextrin An excess of drug was added to 10 mL of b-cyclodextrin aqueous solution (15 mmol L1) in a stoppered conical flask and shaken at room temperature on a magnetic stirrer. At equilibrium after 2 days, aliquots were withdrawn, centrifuged at 4000 rpm V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. for 10 minutes, filtered (0.22-mm pore size, Whatman, UK) and spectrophotometrically assayed for drug content at 285 nm. Phase solubility studies with b-cyclodextrin and citric acid An excess of drug was added to 10 mL aqueous solution containing citric acid (14 mmol L1) and 2 mmol L1 of b-cyclodextrin in a stoppered conical flask and shaken at room temperature on a magnetic stirrer. At equilibrium after 2 days, aliquots were withdrawn, centrifuged at 4000 rpm for 10 minutes, filtered (0.22-mm pore size, Whatman) and spectrophotometrically assayed for drug content at 285 nm. Preparation of ternary complexes Carvedilol, b-cyclodextrin and citric acid ternary complexes were prepared at 1:2:2 molar ratios, respectively, as described in detail below. Physical mixture (PM). For physical mixtures, carvedilol, b-cyclodextrin and citric acid were weighed accurately, mixed thoroughly by trituration in a mortar and sieved through a 0.25-mm sieve. All physical mixtures were stored in a dessicator until further evaluation. Kneaded complex (KC). The kneaded complex of carvedilol, b-cyclodextrin and citric acid was prepared by wetting the physical mixture in a mortar with a minimum volume of ethanol/water mixture (15/85, V/V) and kneading thoroughly with a pestle to obtain a paste, which was then dried under vacuum at room temperature, sieved through a 0.25-mm sieve and stored in a dessicator until further evaluation. Spray dried complex (SDC). A mixture of carvedilol, b-cyclodextrin and citric acid was dissolved in ethanol/water (15/85, V/V). The resultant clear solution was kept for stirring on a magnetic stirrer for 48 hours at room temperature so as to attain complexation equilibrium. Spray drying was carried out using a laboratory scale spray dryer (Jay Instruments and System Pvt. Ltd., India) under the following set of conditions: inlet temperature 112 °C, outlet temperature 55 °C, atomization air pressure 100 kPa, aspiration pressure 2.5 kPa, flow rate 12 mL min1. The powder sample was sieved through a 0.25-mm sieve and stored in a dessicator until further evaluation. Estimation of free and complexed drug The inclusion yield of complexes was determined by dissolving 10 mg of the complex in 10 mL anhydrous acetonitrile for estimation of free drug and another 10 mg of complex in 10 mL acetonitrile/water solution (1:1) for estimation of total drug. The solutions were centrifuged at 4000 rpm for 10 minutes, filtered (0.22-mm pore size, Whatman) and spectrophotometrically assayed for drug content at 285 nm. Differential scanning calorimetry (DSC) DSC studies were carried out using differential scanning calorimetry equipped with an intracooler (DSC 821e, Mettler Toledo, Switzerland). Indium/zinc standards were used V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. to calibrate the DSC temperature and enthalpy scale. The drug and drug complex were hermetically sealed in aluminum pans and heated at a constant rate of 10 °C min1 over a temperature range of 25 to 225 °C; inert atmosphere was maintained by purging nitrogen gas at a flow rate of 50 mL min1. Powder X-ray diffractometry (PXRD) PXRD patterns were recorded on an X-ray diffractometer (PW 1729, Philips, The Netherlands). The samples were irradiated with monochromatized Cu-k radiation and analyzed between 2 to 70 oq. The voltage and current used were 30 kV and 30 mA, respectively. Scanning electron microscopy (SEM) Samples were coated with a thin gold-palladium layer using a sputter coater unit (VG-Microtech, UK) and the surface topography was analyzed with a Cambridge Stereoscan 120 scanning electron microscope (SEM, Cambridge, UK) operated at an acceleration voltage of 15 kV. Fourier transform infrared spectroscopy (FTIR) FTIR spectroscopy was performed on a Fourier Transform Infrared Spectrometer (JASCO, V 5300, Japan). Pellets of the drug sample in KBr were prepared on KBr press (Spectra Lab, India). The spectra were scanned over a wavenumber range of 4000 to 400 cm1. Saturation solubility of the complex Saturation solubility of the complexes was determined by equilibrating an excess of complex in different media such as 0.1 mol L1 HCl (pH 1.2), acetate buffer (pH 4.5), simulated saliva (pH 6.2), and distilled water for 48 hours on a mechanical shaker at room temperature. At equilibrium after 2 days, aliquots were withdrawn, followed by centrifugation at 4000 rpm for 10 minutes, filtration (0.22-mm pore size, Whatman) and a spectrophotometric assay for drug content at 285 nm. Preparation of mouth-dissolving tablets All formulations of pure carvedilol or carvedilol complex (equivalent to 6.25 mg of carvedilol) were compressed into tablets using a 10-station, single rotary, B-tooling tablet machine (M/s Rimek Tabletting Machine, MINI PRESS-I, B-tooling, Karnawati Engineering Ltd, India) using a flat face bevel edged 11-mm die and punch set. Tabletting excipients such as lactose monohydrate, microcrystalline cellulose (Avicel pH 102), mannitol, saccharine, crosspovidone (super disintegrant) or magnesium stearate were added as per the composition given in Table I. Tablets were prepared by the wet granulation method using water as a granulating agent. The prepared tablets were evaluated for mass variation, hardness, friability, content uniformity, disintegration time and in vitro dissolution. V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. Table I. Composition of carvedilol complex tablets Sample No. Component Intragranular portion 1 2 3 4 5 6 7 8 9 Ternary complex obtained by spray drying (6.25 mg carvedilol) Lactose Microcrystalline cellulose Mannitol Saccharine Crospovidone Water (granulating solution) Extragranular portion Crospovidone Magnesium stearate Total mass of tablet 6.00 2.00 150.00 56.81 25.18 27.00 20.00 2.00 4.00 q.s. Mass per tablet (mg) In vitro dissolution studies Dissolution studies were performed using a USP 27 (8) type II dissolution test apparatus (TDT-06P, Electrolab, India) in distilled water. Tablets of different formulations were placed in a dissolution vessel containing 900 mL medium maintained at 37 ± 0.5 °C and stirred at 50 rpm. Samples were collected periodically and were replaced with fresh dissolution medium. Collected samples were filtered through Whatman filter paper. Concentration of carvedilol was determined spectrophotometrically at 285 nm. Stability studies The accelerated stability study of prepared tablets was carried out as per ICH guidelines at 40 °C/75 % RH for up to 3 months. Samples were removed and evaluated for mass variation, hardness, friability, content uniformity, disintegration time, dissolution, PXRD and DSC studies. RESULTS AND DISCUSSION Phase solubility studies The phase-solubility diagram for complex formation between carvedilol and b-cyclodextrin is presented in Fig. 1. According to the Higuchi and Connors classification (9), the diagram shows the AN type curve where the solubility increases linearly with cyclodextrin concentration and further deviates negatively at higher cyclodextrin concentrations. The interaction mechanism for the AN type curve is complicated because of a significant contribution of solute-solvent interaction to the complexation. The negative V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. Carvedilol solubility ´ 10 5 (mol L1 ) 12 10 8 6 4 2 0 0 1 2 3 5 4 2 1 Complexing agent ´ 10 (mol L ) 6 Carvedilol solubility ´ 10 3 (mol L1 ) Fig. 1. Phase solubility diagram of carvedilol in solutions containing: a) varying contretation of b-cyclodextrin (t) and varying concentration of citric acid containing 2 mmol L1 of b-cyclodextrin (n). deviation from linearity may be associated with cyclodextrin induced changes in the dielectric constant of the aqueous complexation media, changes in complex solubility or self-association of cyclodextrin molecules. It can be seen from the curve, that the apparent solubility of carvedilol increased linearly due to the formation of a soluble inclusion complex between carvedilol and b-cyclodextrin. Earlier Wen et al. (10) reported a similar AN type curve and further concluded from the fluorescence study that carvedilol forms a 1:2 complex with b-cyclodextrin. The phase solubility diagram of carvedilol with b-cyclodextrin and citric acid (Fig. 1) illustrates the AN type of curve. The addition of citric acid along with b-cyclodextrin resulted in an increase in solubility of carvedilol to a much greater extent compared to complexation of the drug with b-cyclodextrin alone. This may be attributed to the fact that organic hydroxyl acids such as citric acid are able to enhance the solubility of the basic guest molecule both by forming a salt and by increasing the stability constant of the complex (11). Thus the addition of citric acid results in formation of carvedilol citrate salt (6). Estimation of free and complexed drug content The complexation efficiency, i.e., the inclusion yield of the drug in b-cyclodextrin was determined based on the differential solubility of free drug and complexed drug (12). The amount of complexed drug was the highest in the case of the complex obtained by the spray drying method (83.2 ± 0.6 %) compared to the kneaded complex (68.7 ± 1.1 %) and physical mixture (3.4 ± 1.0 %) (n = 3). This indicated that the drug in the physical mixture remained essentially in the un-complexed form. Hence, complexation by spray drying was found to be a highly efficient method to produce the carvedilol-b-cyclodextrin-citric acid ternary complex. Differential scanning calorimetry DSC was used to identify the inclusion ternary complex between the drug, b-cyclodextrin and citric acid. As seen from Fig. 2a, the DSC thermogram of carvedilol alone V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. showed a sharp endothermic peak at 118 °C, corresponding to the melting point of the crystalline form of carvedilol. The physical mixture showed separate endotherms corresponding to carvedilol (118 °C), citric acid (7580 °C) and b-cyclodextrin, indicating no complex formation. In case of the kneaded complex, a broad endotherm was obtained around 120 °C. The thermogram of the inclusion ternary complex prepared by the spray drying method shifted towards lower temperature showed a broad endotherm between 90 and 100 °C. Lower temperature of the inclusion ternary complex can be attributed to the melting point depression in the complex. Similar conditions were found when felodipine and carbamazepine were complexed with b-cyclodextrin, which resulted in the lowering of the endothermic peak of the complexed drug (13). Also in the case of the b) a) d c EXO mW b Intensity a d 25 225 68 51 34 17 2q (degrees) 2 Temperature (°C) c) a b %T d 4000 400 Wavenumber (cm ) 1 Fig. 2. a) Differential scanning calorimetry, b) X-Ray powder diffraction patterns and c) FT-IR spectra of: a pure drug, b physical mixture, c complex obtained by kneading, d complex obtained by spray drying. 127 V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. complex obtained by spray drying, it can be concluded that amorphization of the drug occurred in the presence of b-cyclodextrin and citric acid and trapping of carvedilol inside the b-cyclodextrin cavity, leading to the broad nature of the endotherm. Powder X-ray diffractometry The PXRD graphs for pure drug, physical mixture, kneaded complex and complex obtained by sprey drying are shown in Fig. 2b. Peaks for crystallinity were observed in pure drug at 2q values of 6, 12, 14, 15, 18.5, 25, 27 and 28°. The PXRD of complexes prepared by physical mixing and kneading revealed the presence of the uncomplexed crystalline nature of the drug. However, it was interesting to note that these peaks were suppressed or absent in the complex obtained by spraying drying indicating amorphization of the drug and complexation of carvedilol with b-cyclodextrin. These results of PXRD were strongly supported by the above DSC observations. Fourier transform infrared spectroscopy Fig. 2c shows the infrared spectra of pure drug, physical mixture, kneaded complex and ternary complex obtained by spray drying. Carvedilol showed characteristic peaks at 3346.8 cm1 (O-H and N-H stretching vibration peaks merged together), 2924.35 cm1 (C-H stretching vibrations), 1591.42 cm1 (N-H bending vibrations) and 1255.77 cm1 (O-H bending and C-O stretching vibrations). The intensity and shape of these bands changed dramatically for the inclusion ternary complex compared to the physical mixture and pure drug. The complex showed broadening of O-H and N-H stretching vibrations peaks (3383.45 cm1). Differences were found in the 14801600 cm1 region, which were attributed to skeleton vibrations of the C=C bonds in the aromatic ring. In the low frequency region (1000400 cm1), all the spectra were almost unchanged, indicating that overall symmetry of the molecule was not significantly affected even though the vibration and bending of the drug were restricted due to the formation of an inclusion complex. Scanning electron microscopy The microphotographs of pure drug, kneaded and spray dried ternary complex are shown in Fig. 3. The pure drug appeared to be in the crystalline form. In case of the ternary complex obtained by spray drying, significant changes in particle shape and surface topography were observed. Particles appeared as irregularly shaped agglomerates a) b) Fig. 3. Scanning electron microscope photographs: a) pure drug, b) complex obtained by spray drying. 128 V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. Table II. Saturation solubility Physical mixture (mg mL1)a 0.092 ± 0.019 0.548 ± 0.009 0.826 ± 0.423 0.076 ± 0.003 Ternary complex Ternary complex obtained by knead- obtained by spray ing (mg mL1)a drying (mg mL1)a 0.650 ± 0.090 0.580 ± 0.075 1.048 ± 0.016 0.258 ± 0.092 0.927 ± 0.060 0.614 ± 0.033 1.329 ± 0.126 0.433 ± 0.011 Solvent Distilled water HCl (pH 1.2) Acetate buffer (pH 4.5) Pure drug (mg mL1)a 0.008 ± 0.003 0.567 ± 0.028 0.707 ± 0.044 Simulated saliva (pH 6.2) 0.074 ± 0.003 Mean ± SD, n = 3. with surface smoothness indicating an amorphous nature. These results support the PXRD and DSC studies. Saturation solubility of complexes Saturation solubility of the pure drug and complexed drug prepared by different methods in different buffer media is shown in Table II. Carvedilol is almost insoluble in water, with saturation, solubility of (8.5 ± 2.6) ´ 103 mg mL1. A significant increase (110-fold) in solubility in distilled water was obtained in the case of the ternary complex obtained by spray drying. This may be attributed to the formation of the ternary complex of the drug with b-cyclodextrin and citric acid. Also, the spray-drying method has greater efficiency for inclusion complex formation than the kneading method. It also resulted in the amorphous nature of the complex, as evidenced in DSC, PXRD and SEM studies. The saturation solubility of the spray dried drug was also determined and found to be (9.2 ± 1.9) ´ 103 mg mL1. Thus, mere spray drying of the drug does not bring about an increase in solubility of the drug as obtained in the case of the ternary complex obtained by spray drying. Physical properties and dissolution of tablets The tablets containing pure drug and the ternary complex obtained by kneading and spray drying were prepared using the wet granulation technique. Tablets were analyzed for mass variation, friability, disintegration time and drug content (Table III). The in vitro drug dissolution studies of the tablet with complexed carvedilol obtained by Table III. Physical properties of tablets Sample No. Pure drug Complex obtained by spray drying Mass (mg)a 151.7 ± 1.1 150.8 ± 1.2 Hardness (N)a 40.2 ± 2.2 40.2 ± 2.0 Friability (%) 0.719 0.068 Disintegration time (s) 22 24 Drug (%) 102.7 ± 1.5 96.3 ± 2.4 Mean ± SD, n = 3. V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132. Drug releases (%) 100 80 60 40 20 0 10 20 30 40 50 60 70 Time (min) Fig. 4. Dissolution profile of the pure drug tablet (t) and the tablet with complex obtained by spray drying (n) (mean ± SD, n = 6). spray drying and pure carvedilol tablet were carried out in distilled water (shown in Fig. 4). A significant improvement in the dissolution profile of tablets prepared from the carvedilol complex was observed as compared to the pure drug tablet. 100 and 30 % release, resp. was obtained in 5 minutes in the case of tablets with the complex and pure drug tablets. These results support the saturation solubility study. Since 100 % drug release was achieved in 5 minutes this makes it suitable as a mouth-dissolving tablet. Stability study Stability studies were carried out for the tablets with the complex obtained by spray drying by exposing them to 40 °C at 75 % relative humidity. Drug content, disintegration time, dissolution, XRD and DSC studies were carried out at the end of 3 months and compared to the day zero. No significant changes were observed in drug content, dissolution and disintegration of the tablet with the complex obtained by spray drying after 3 months. Also, the PXRD and DSC patterns of the complex powder obtained by spray drying recorded after 3 months gave identical patterns (data not shown) to the initial ones. CONCLUSIONS Phase solubility studies of carvedilol with b-cyclodextrin showed that the AN type of curve was obtained, which indicated that the complex was formed in a 1:2 molar ratio. Phase solubility of the drug with b-cyclodextrin and citric acid showed higher solubilization efficiency. The ternary complex of carvedilol-b-cyclodextrin-citric acid was formed in a molar ratio of 1:2:2. Preparation of carvedilol-b-cyclodextrin-citric acid ternary complex by spray drying resulted in effective complexation with significant improvement in aqueous solubility compared to the physical mixture and kneaded complex. The in vitro dissolution studies of the tablets prepared from the ternary complex of carvedilol obtained by spray drying showed 100 % release in 5 minutes. In the stability study, no significant changes were recorded with respect to XRD, DSC, drug content, disintegration and dissolution over a period of 3 months, indicating a stable complex. Since 100 % release of the drug from the tablet was obtained within 5 minutes, the approach is suitable for the formulation of mouth-dissolving tablets of carvedilol. V. Pokharkar et al.: Ternary complexation of carvedilol, b-cyclodextrin and citric acid for mouth-dissolving tablet formulation, Acta Pharm. 59 (2009) 121132.
Acta Pharmaceutica – de Gruyter
Published: Jun 1, 2009
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