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Modification and Characterization of Gellan Gum
Carbohydrate polymers have been widely used in the formulations of pharmaceutical solid dosage forms becasuse of their biodegradability and lack of oral toxicity. Gellan gum (commercially available as Gelrite or Kelcogel) is a gel-forming polysaccharide produced by the microbe Sphingomonas elodea (formerly Pseudomonas elodea) and approved for food use by the US Food and Drug Administration in 1992 (1, 2). The gellan polymer consists of monosaccharide α-L-rhamnose, β-D-glucuronic acid, and β-D-glucose in a molar ratio of 1:1:2 linked to form a linear primary structure (3).
The native polymer is a high acyl gellan containing O-5-acetyl and O-2-glyceryl groups on the (1-3)-linked glucose residue. When exposed to alkali at high temperatures, both acyl groups are hydrolyzed, and the deacylated form of low acyl gellan is obtained. Low acyl gellan dissolves in water at temperatures >90 °C. The low acyl solution forms gels in the presence of cations when cooled to gelling temperature (4). High acyl gellan solutions gel at much higher temperatures than do low acyl gellan solutions. High acyl gellan gels are very weak because the bulky acetyl and glyceryl groups prevent close association between gellan polymer chains in a double-helix formation and hinder compact packing of the cross-linked double helix. Aqueous solutions of gellan gum form gels on warming to body temperature and in the presence of cations (5). The mechanism of gelation involves the formation of double helical junction zones followed by aggregation of the double helical segments to form a three-dimensional network by complexation with cations and hydrogen bonding with water (6–8).
Gellan gum has been widely accepted for various pharmaceutical applications, although it has not been explored as a disintegrant in a tablet formulation. Gellan gels have been studied for application in ophthalmic drug delivery (9–11), oral sustained delivery (12, 13), beads (14), sustained delivery beads (15), controlled-release hydrogel with scleroglucan (16), and floating in situ gelling (17).
Since the implementation of polymeric materials in the field of pharmaceutical technology, numerous attempts have been undertaken to modify their physical and chemical properties and apply them in drug formulations. These approaches include cross-link polymers through covalent bonds using chemical and physical means. Formaldehyde, gluteraldehyde, and epichlorohydrin are among the most commonly used chemical means of cross-linking. Ultraviolet (18, 19) and gamma radiations (20) and temperature variations represent physical methods of cross-linking. Modifying temperature is one of the most favorable approaches because it avoids both the application of harsh chemical materials suitable for large-scale production and does not require a diversity of equipment and methods in the application (21).
Microwave technology is a growing interest in the pharmaceutical industry. Various applications include drying pharmaceutical excipients (22), granules (23-26), film coats (27), sterilization of ampules (28), and formulation of controlled-release beads (29).
in which f is the frequency of microwave, E is the electric field, E 0 is the dielectric constant of free space, Er is the dielectric constant of molecule, and tanδ is the loss tangent. The dielectric constant of a molecule is related to the polarizability of its constituent material, and the loss tangent is regarded as a measure of molecular interaction. Polar materials with a high level of molecular interaction are expected to absorb more energy, giving rise to a more intense molecular vibration and heat. The state of molecular interaction of an object may change its physicochemical properties over time (30, 31). Microwave heating offers rapid heat transfer, high speed, energy penetration, selective energy absorption, and instantaneous electronic control. It also requires compact equipment, fewer British thermal units (BTUs) and is environmentally clean.
Cross-linking natural macromolecules with microwave energy might solve toxicity problems resulting from remaining traces of chemical cross-linking agents and in vivo biodegradation products of chemically cross-linked macromolecules. This approach may help identify new uses for an existing excipient, which is relatively inexpensive and simpler than developing a new excipient.
The authors modified gellan gum by microwave technology and compared its various physicochemical properties with those of pure gellan gum. Confirmation of physical modification was carried out with differential scanning calorimetry (DSC) and X-ray diffractomy. Chemical changes in the modified material were analyzed with Fourier transfer infrared (FT-IR) spectroscopy. Optimization of the amount of distilled water and time of exposure in microwave was carried out using a 32 full-factorial design. Tablets of lactose and dibasic calcium phosphate that also contained modified gellan gum were prepared, and the effect on tablet disintegration time, hardness, and friability were measured.
Materials and methods
Materials. Gellan gum was received as a generous gift sample from CP Kelco (Mumbai, India). Dibasic calcium phosphate (DCP) IP, lactose IP, polyvinyl pyrollidone (PVP), and talc IP were used as received. All other solvents and chemicals were of AR grade. Deionized double distilled water was used throughout the study.
Swelling ratio. The experiment was carried out in a 100-mL stopper graduated cylinder. The initial bulk volume of 1 g of MGG was noted and water was added in sufficient quantity to yield a 100-mL uniform dispersion. The sediment volume of the swollen mass was noted after 24 h at room temp. The swelling ratio was calculated by taking the ratio of the swollen volume to the initial bulk volume (32).
DSC. DSC thermograms were obtained (Model TA-60, Shimadzu, Japan). About 2 mg of sample were scanned in a hermetically sealed standard aluminum pan and heated over a 50–300 °C temperature range at a rate of 10 °C/min under constant purging of nitrogen at 40 mL/min. An empty sealed aluminum pan was used as a reference. The characteristic peaks and specific heat of the melting endotherm were recorded.
FT-IR spectroscopy. About 2% (w/w) of PGG and MGG samples, with respect to the potassium bromide (KBr) disk, was mixed with dry KBr (FT-IR grade). The mixture was ground into a fine powder before compressing into a disk. Each disk was scanned at a resolution of 4 cm–1 over a wave number region of 400–4000cm–1 using an FT-IR spectrometer (Spectrum GX FTIR system, Perkin Elmer, Waltham, MA). The characteristic peaks of IR transmission spectra were recorded.
X-ray powder diffractometry. These studies were performed with samples of PGG and MGG. The samples were filled into an aluminum sample holder and exposed to Cu K-α radiation (40 kV x 40 mA) in a wide-angle X-ray powder diffractometer (model X'Pert MPD, Philips Analytical, The Netherlands). Each sample was scanned in a continuous mode with the diffraction angle, 2θ, increasing from 5 < 2θ <100.
Particle size. Average particle size of PGG and MGG was measured with optical microscopy.
Angle of repose. The authors used the fixed-funnel and free-standing method to measure the angle of repose. The measurement was made in triplicate and the mean angle of repose was calculated.
Bulk and tapped density. Bulk and tapped densities of PGG and MGG were determined. The powder was placed inside the measuring cylinder of a tapped density apparatus, and the bulk volume was recorded. The samples were subjected to 200 taps, and the tapped volume was recorded. The bulk and tapped densities were computed.
Carr's index and Hausner ratio. The Carr's index and Hausner ratio of PGG and MGG were calculated by the equations provided in Aulton 2002 (33).
Disintegration time. The time required for disintegration of six tablets per batch was carried out in a USP disintegration test apparatus (model ED2L, Electrolab, Mumbai, India) containing 900 mL distilled water at 37 ± 0.50 °C.
Hardness and friability test. A hardness tester (Monsanto type) was used to measure tablet hardness (n = 5). Friability was evaluated as the percentage weight loss of 20 tablets tumbled in a friabilator (USP XXIII, model EF2, Electrolab, Mumbai) for 4 min at 25 rpm. The tablets were dedusted, and the loss in weight caused by fracture or abrasion was recorded as percentage friability.
Results and discussion
A gelation of PGG is a function of polymer concentration, temperature, and the presence of monovalent and divalent cations in solution. At low temperature, gellan forms an ordered helix of double strands. At high temperatures, single-stranded polysaccharides occur, which significantly reduce the viscosity of solution. PGG possesses high viscosity and gelling ability, which can be useful as release retardants in the preparation of a controlled-release formulation.
In our preliminary studies, we observed that the viscosity of PGG was reduced when it was exposed to high temperature. Hence, a modified form was prepared by exposing PGG in a microwave oven at higher temperature for different time periods. The results of physical changes attributed to PGG upon modification shows a marked decrease in viscosity as well as gelling ability. The MGG possesses excellent swelling property but lesser viscosity; this may be a result of the formation of single-stranded polysaccharides at higher temperature (35–37).
in which Y is the dependent variable, b 0 is the arithmetic mean response of the nine runs, and b1 is the estimated coefficient for the factor X 1. The main effects (X 1 and X 2) represent the average result of changing one factor at a time from its low to high value. The interaction terms (X 1 X 2) show how the response changes when two factors are simultaneously changed (38). Factorial design was calculated with the help of Microsoft Excel 2007.
in which DF is degree of freedom and F is Fischer's ratio.
A higher value of X 2 (time of exposure in microwave) was observed as compared with X 1 (amount of distilled water) in all three equations. It can be concluded that time of exposure in microwave (X 2) had more effect on swelling ratio than the amount of distilled water (X 1) in the modification of PGG. In all three media, X 1 and X 2 carried positive signs, meaning there was a positive effect of both factors on the swelling ratio. Batch I showed maximum swelling in all three media as compared with other batches. Therefore, Batch I was selected as the optimized batch and was used for further study. Higher values of X 1 (amount of distilled water) and X 2 (time of exposure in microwave) favors swelling ratio of MGG in distilled water, HCl, and phosphate buffer.
Lactose tablets showed relatively faster disintegration (Batch A1) than DCP tablets (Batch B1), which may be attributed to an increased water uptake by lactose tablets. The slightly higher DT of DCP tablets compared with lactose tablets may be attributed to higher crushing strength and poor aqueous solubility (41). Results reveal that tablets containing an insoluble excipient (DCP) and MGG (Batch B2) showed substantial decrease in DT as compared with Batch B1, which contained no disintegrant. For lactose tablets with and without MGG, there was a decrease in DT. Lactose is a water-soluble excipient, and hence it works as an auxiliary disintegrant. A comparison of batches A2 and B2 shows that there was more decrease in DT in batch B2. Therefore, MGG led to excellent results in the presence of a hydrophobic excipient (DCP).
The following results were concluded when PGG was treated in microwave:
MGG was explored as a disintegrant in lactose and DCP tablets. MGG functions as a disintegrant in tablet formulation and shows excellent DT with DCP (hydrophobic excipient) as compared with lactose (hydrophilic excipient).
The areas in which further work can be conducted include using a fluid-bed dryer or spray dryer for preparing a modified excipient. MGG can be used in combinations and ratios of presently accepted super disintegrating agent such as crospovidone, sodium starch glycolate, and croscarmellose.
Results of this study show that modified gellan gum can be used as a disintegrant in tablet formulation. Only physical modification was carried out in the microwave oven. Modified gellan gum shows excellent swelling capacity, flow property, and compressibility.
Dhiren P. Shah* is an assistant professor in the department of pharmaceutics at CK Pithawala Institute of Pharmaceutical Science and Research, Via Magdalla Port, Nr. Malvan Mandir, Dumas Road, Surat, Gujrat, India 395 007, tel. +91 261 6587286, fax: +91 261 272399. Girish K. Jani is a principal at SSR College of Pharmacy (UT of Dadra and Haveli, India).
*To whom all correspondence should be addressed.
Submitted: Sept. 10, 2008. Accepted: Oct. 1, 2008.
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