Cross-linked high amylose starch for controlled release of drugs: recent advances

Synthesis and release studies on amylose-based ester prodrugs of fenamic acid NSAIDs

Aim: To achieve colon-targeted release of mefenamic acid from its ester-linked amylose prodrugs.Materials & methods: The prodrug was characterized by 1H NMR and IR spectroscopy. Drug activation and release profile was studied in enzyme enriched simulated physiological media via UV-vis spectroscopy and was validated with HPLC analysis. ELISA assay was employed for evaluating the % inhibition of COX-1 and COX-2 inhibition at different concentrations of the prodrug preincubated with ester and/ or amylose hydrolyzing enzymes. SEM studies further validated the performance of the prodrug under simulated physiological conditions.Results: Pancreatin was essential for the prodrug activation in SIM to make the ester bonds in prodrug vulnerable to hydrolysis by esterase. This evidence was confirmed by drug release studies, HPLC analysis, ELISA assay and SEM investigation where the ester conjugated prodrug showed marked stability in physiological media only to get activated in the presence of amylose degrading enzyme.Conclusion: Ester linked amylose-mefenamic acid conjugate showed both enzyme responsive activation and release in SIM.

Anionic and Ampholytic High-Amylose Starch Derivatives as Excipients for Pharmaceutical and Biopharmaceutical Applications: Structure-Properties Correlations

Many chemical modifications of starch are realized in organic (mostly methanol) phase, allowing high degrees of substitution (DS). Some of these materials are used as disintegrants. To expand the usage of starch derivative biopolymers as drug delivery system, various starch derivatives obtained in aqueous phase were evaluated with the aim to identify materials and procedures which would generate multifunctional excipients providing gastro-protection for controlled drug delivery. Chemical, structural and thermal characteristics of anionic and ampholytic High Amylose Starch (HAS) derivatives under powder (P), tablet (T) and film (F) forms were evaluated by X-ray Diffraction (XRD), Fourier Transformed Infrared (FTIR) and thermogravimetric analysis (TGA) methods and correlated with the behavior of tablets and films in simulated gastric and intestinal media. At low DS, the HAS carboxymethylation (CMHAS) in aqueous phase, generated tablets and films that were insoluble at ambient conditions. The CMHAS filmogenic solutions, with a lower viscosity, were easier to cast and gave smooth films without the use of plasticizer. Correlations were found between structural parameters and the properties of starch excipients. Compared to other starch modification procedures, the aqueous modification of HAS generated tunable multifunctional excipients that may be recommended for tablets and functional coatings for colon-targeted formulations.

Physiologically Based Dissolution Testing in a Drug Development Process-a Case Study of a Successful Application in a Bioequivalence Study of Trazodone ER Formulations Under Fed Conditions

Development of generic extended-release (ER) formulations is challenging. Especially under fed conditions, the risk of failure in bioequivalence trials is high because of long gastric residence times and susceptibility to food effects. We describe the development of a generic trazodone ER formulation that was aided with a biorelevant dissolution evaluation. Trazodone hydrochloride 300-mg monolithic matrix tablets were dissolved both in USP and EMA compliant conditions and in the StressTest device that simulated both physicochemical and mechanical conditions of the gastrointestinal passage. The final formulation was tested against the originator, Trittico XR 300 mg, in a randomized cross-over bioequivalence trial with 44 healthy volunteers, in agreement with EMA guidelines. Initially developed formulations dissolved trazodone similarly to the originator under standard conditions (f2 factor above 50), but their dissolution kinetics differed significantly in the biorelevant tests. The formulation was optimized by the addition of low-viscosity hypromellose and mannitol. The final formulation was approved for the bioequivalence trial. Calculated Cmax were 1.92 ± 0.77 and 1.92 ± 0.63 [μg/mL], AUC0-t were 27.46 ± 8.39 and 29.96 ± 9.09 [μg∙h/mL], and AUC0-∞ were 28.22 ± 8.91 and 30.82 ± 9.41 [μg∙h/mL] for the originator and test formulations, respectively. The 90% confidence intervals of all primary pharmacokinetic parameters fell within the 80-125% range. In summary, biorelevant dissolution tests supported successful development of a generic trazodone ER formulation pharmaceutically equivalent with the originator under fed conditions. Employment of biorelevant dissolution tests may decrease the risk of failure in bioequivalence trials of ER formulations.

Robustness of Controlled Release Tablets Based on a Cross-linked Pregelatinized Potato Starch Matrix

The aim of this study was to evaluate the potential of a cross-linked pregelatinized potato starch (PREGEFLO® PI10) as matrix former for controlled release tablets. Different types of tablets loaded with diprophylline, diltiazem HCl or theophylline were prepared by direct compression of binary drug/polymer blends. The drug content was varied from 20 to 50%. Two hydroxypropyl methylcellulose grades (HPMC K100LV and K100M) were studied as alternative matrix formers. Drug release was measured in a variety of release media using different types of experimental set-ups. This includes 0.1 N HCl, phosphate buffer pH 6.8 and water, optionally containing different amounts of NaCl, sucrose, ethanol or pancreatin, fasted state simulated gastric fluid, fed state simulated gastric fluid, fasted state simulated intestinal fluid, fed state simulated intestinal fluid as well as media simulating the conditions in the colon of healthy subjects and patients suffering from Crohn's disease. The USP apparatuses I/II/III were used under a range of operating conditions and optionally coupled with the simulation of additional mechanical stress. Importantly, the drug release kinetics was not substantially affected by the investigated environmental conditions from tablets based on the cross-linked pregelatinized potato starch, similar to HPMC tablets. However, in contrast to the latter, the starch-based tablets roughly kept their shape upon exposure to the release media (they "only" increased in size) during the observation period, and the water penetration into the systems was much less pronounced. Thus, the investigated cross-linked pregelatinized potato starch offers an interesting potential as matrix former in controlled release tablets.

Silk Fibroin as a Novel Alcohol-Resistant Excipient for Sustained-Release Tablet Formulation

Concomitant intake of alcoholic beverages with sustained-release oral formulations may potentially lead to dose dumping. Alcohol-resistance testing is currently a requirement for the manufacturers by regulatory authorities. Silk fibroin produced by silkworm Bombyx mori is suggested in this work as a potential alternative to a narrow spectrum of alcohol-resistant excipients. Oxycodone HCl, tramadol HCl, and flurbiprofen were selected as model drugs and formulated with regenerated silk fibroin either in the form of an amorphous solid dispersion or as a physical mixture and compressed into tablets. Preliminary compactability and tampering-resistance studies were performed. The ethanol-resistance was tested in media containing 5%, 10%, 20%, or 40% (v/v) ethanol concentration. Drug release profiles were compared using f₂ similarity factor. Good mechanical tampering-resistance (tensile strength of 14.6 MPa at 400 MPa compression pressure) was obtained for tablets compressed from physical mixture. Tablets compressed from amorphous solid dispersion had lower tensile strength (2.2 MPa) but showed chemical tampering-resistance to extraction by pure ethanol (7.1% of oxycodone HCl after 24 h). Drug release is controlled predominantly by swelling and diffusion. With an increasing ethanol concentration in release medium, the tablets swelled less, resulting in a slower release. This trend was similar for all tested drugs and for both physical states formulations. No dose dumping occurred in the presence of ethanol; therefore, silk fibroin could be considered as an alternative alcohol-resistant excipient for sustained release application.

Ampholytic starch excipients for high loaded drug formulations: Mechanistic insights

Ampholytic starch derivatives are proposed as a new class of excipients carrying simultaneously anionic carboxymethyl (CM) and cationic aminoethyl (AE) groups on starch (St) polymeric chains. Three different types of derivatives were obtained by using the same reagents and varying only the order of their addition in the reaction medium: in one step method (OS) the two reactants were added simultaneously, whereas in two steps method (TS) either CMSt or AESt were prepared separately in the first step, followed by subsequent addition of the second reactant. It was found that all ampholytic derivatives were able to generate monolithic tablets by direct compression and allowed 60% loading of acidic (Acetylsalicylic acid), basic (Metformin), zwitterion (Mesalamine) or neutral (Acetaminophen) as drug models. The in vitro dissolution tests followed for 2 h in SGF and then in SIF, showed that the mentioned starch derivatives were stabilized by self-assembling and generated matrices able to control the release of drugs for about 24 h. The addition order of reagents has an impact on ampholytic starch properties offering thus a high versatility of this new class of starch excipients that can be tailored for challenging formulations with high dosages of several drugs.

Carboxymethyl Starch Excipients for Drug Chronodelivery

Carboxymethyl starch (CMS) is a pH-responsive excipient exhibiting also interesting properties for applications in delayed drug delivery systems. This work was aimed to investigate the release properties of monolithic and dry-coated tablets based on ionic sodium CMS and on protonated CMS, formulated with three model tracers: acetaminophen, acetylsalicylic acid (ASA), and sodium diclofenac. The sodium or protonated CMS were obtained from the same CMS synthesis by controlling the final pH of reaction media. The two forms of CMS were confirmed by the Fourier transform infrared spectroscopy. The in vitro dissolution profiles for monolithic and double core tablets were different and allowed a better understanding of characteristics of the two excipient forms. It was found that the protonated CMS exhibited a better stability in simulated gastric fluid in comparison to its sodium salt in monolithic dosage forms, whereas both excipients afforded a complete gastric protection of drugs when formulated as dry-coated dosages. Determination of water uptake and erosion rate of monolithic matrices based on the two CMS forms showed different mechanisms involved in the delivery of the three model active molecules in simulated intestinal media. When pancreatic enzymes were added in dissolution media, the drug release was accelerated showing that CMS is still a substrate for alpha-amylase. Both sodium and protonated starch excipients, formulated as dry-coated dosages, afforded a good gastro-protection and allowed a drug chronodelivery at various intervals up to 4-5 h. They could be considered as an alternative for delayed delivery and a solvent-free coating procedure.

Starch materials as biocompatible supports and procedure for fast separation of macrophages

Different starch derivatives were evaluated as supports for attachment and recovery of macrophages (RAW 264.7 line). Gelatinized starch (G-St), acetate starch (Ac-St), carboxymethyl starch and aminoethyl starch were synthesized and characterized by FTIR, ¹H NMR, SEM and static water contact angle. These polymers are filmogenic and may coat well the holder devices used for macrophage adhesion. They also present a susceptibility to mild hydrolysis with alpha-amylase, liberating the adhered macrophages. Cell counts, percentage of dead cells and level of tumor necrosis factor (TNF-α) were used to evaluate the possible interaction between macrophages and starch films. The high percentage of cell adhesion (90-95% on G-St and on Ac-St) associated with enzymatic detachment of macrophages from film-coated inserts, resulted in higher viabilities compared with those obtained with cells detached by current methods scrapping or vortex. This novel method allows a fast macrophage separation, with excellent yields and high viability of recovered cells.

Synthesis and evaluation of the structural and physicochemical properties of carboxymethyl pregelatinized starch as a pharmaceutical excipient

A pregelatinized starch (PGS) was derivatized with sodium chloroacetate (SCA) in alcoholic medium under alkaline condition to produce carboxymethyl pregelatinized starch (CMPGS) with various degrees of substitution (DS). Influence of the molar ratio of SCA to the glucopyranose units (SCA/GU), reaction time, temperature and the amount of sodium hydroxide on the degree of substitution (DS) and the reaction efficiency (RE) was studied. An optimal concentration of 30% of NaOH, for a reaction time of 1 h at 50 °C and molar ratio (SCA/GU) equal to 1.0, yielded an optimal DS of 0.55 and a RE of 55%. SEM micrographs revealed that the carboxymethylation assigned the structural arrangement of CMPGS and caused the granular disintegration. Wide angle diffraction X-ray (XRD) showed that the crystallinity of starch was obviously varied after carboxymethylation. New bands in FTIR spectra at 1417 and 1603 cm(-1) indicated the presence of carboxymethyl groups. The solubility and viscosity of CMPGS increased with an increase in the degree of modification. In order to investigate the influence of DS on physical and drug release properties, CMPGS obtained with DS in the range of 0.12-0.55 was evaluated as tablet excipient for sustained drug release. Dissolution tests performed in phosphate buffer (pH 6.8), with Ibuprofen as drug model (25% loading) showed that CMPGS seems suitable to be used as sustained release excipient since the drug release was driven over a period up to 8 h. The in vitro release kinetics studies revealed that all formulations fit well with Korsmeyer-Peppas model and the mechanism of drug release is non-Fickian diffusion.

Preparation and characterization of pullulanase debranched starches and their properties for drug controlled-release

Linear short amylose and glucan tend to align and aggregate to form hydrogels that hold less water. The drug release properties of debranched starch based tablets can be controlled by the pullulanase modification conditions.

Characterization of different substituted carboxymethyl starch microgels and their interactions with lysozyme

A carboxymethyl starch (CMS) microgel system was prepared for the control of uptaking and releasing proteins (lysozyme). The physicochemical properties of microgels in various degrees of substitution (DS) were determined by thermal gravimetric analysis (TGA), swelling degree, and rheological analysis. The microgel particle size mostly ranged from 25 µm to 45 µm. The result obtained from the TGA studies indicated that carboxymethylation decreased the thermal stability of starch, but crosslinking increased the thermal stability of CMS. The CMS microgels showed typical pH sensitivity, and the swelling degree of microgel increased with the increasing of DS and pH, because of the large amounts of carboxyl group ionization. The samples (2.25%) could behave as viscoelastic solids since the storage modulus was larger than the loss modulus over the entire frequency range. The protein uptake increased with increasing pH and DS at low salt concentration. The optimal pH shifted to lower pH with increasing ionic strength. The saturated protein uptake decreased with increasing ionic strength at each pH. The protein was easily released from the microgel with high pH and high salt concentration.

High-amylose sodium carboxymethyl starch matrices: development and characterization of tramadol hydrochloride sustained-release tablets for oral administration

Substituted amylose (SA) polymers were produced from high-amylose corn starch by etherification of its hydroxyl groups with chloroacetate. Amorphous high-amylose sodium carboxymethyl starch (HASCA), the resulting SA polymer, was spray-dried to obtain an excipient (SD HASCA) with optimal binding and sustained-release (SR) properties. Tablets containing different percentages of SD HASCA and tramadol hydrochloride were produced by direct compression and evaluated for dissolution. Once-daily and twice-daily SD HASCA tablets containing two common dosages of tramadol hydrochloride (100 mg and 200 mg), a freely water-soluble drug, were successfully developed. These SR formulations presented high crushing forces, which facilitate further tablet processing and handling. When exposed to both a pH gradient simulating the pH variations through the gastrointestinal tract and a 40% ethanol medium, a very rigid gel formed progressively at the surface of the tablets providing controlled drug-release properties. These properties indicated that SD HASCA was a promising and robust excipient for oral, sustained drug-release, which may possibly minimize the likelihood of dose dumping and consequent adverse effects, even in the case of coadministration with alcohol.

The design of controlled-release formulations resistant to alcohol-induced dose dumping--a review

The concomitant intake of alcoholic beverages together with oral controlled-release opioid formulations poses a serious safety concern since alcohol has the potential to alter the release rate controlling mechanism of the dosage form which may result in an uncontrolled and immediate drug release. This effect, known as alcohol-induced dose dumping, has drawn attention of the regulatory authorities. Thus, the Food and Drug Administration (FDA) recommends that in vitro drug release studies of controlled-release dosage forms containing drugs with narrow therapeutic range should be conducted in ethanolic media up to 40%. So far, only a limited number of robust dosage forms that withstand the impact of alcohol are available and the development of such dosage forms is still a challenge. This review deals with the physico-chemical key factors which have to be considered for the preparation of alcohol-resistant controlling dosage forms. Furthermore, appropriate matrix systems and promising technological strategies, which are suitable to prevent alcohol-induced dose dumping, are discussed.

NMR imaging study of cross-linked high-amylose starch tablets — The effect of drug loading

NMR imaging techniques were used to study the effect of drug loading in cross-linked high-amylose starch tablets. The tablets contained acetaminophen with loading levels from 10 to 40 wt%. The absolute amount of the drug released increased with a larger amount of drug loading, but the percentages of drug released had only minor differences for the different tablets, probably due to the rapid formation of a gel layer for all the tablets, which slowed down drug release significantly. The release of drugs from the tablets in all cases is dominated by a diffusion mechanism before the disappearance of the dry core of the tablets. Radial and axial swelling and water uptake were found to increase with the amount of drug loading. The diffusion rates of water were comparable at the initial stage for all the tablets with different loadings, but became faster later for the tablets with higher amounts of drug loading as water diffusion may be facilitated by the hydrophilicity of the drug.

Polymeric plant-derived excipients in drug delivery

Drug dosage forms contain many components in addition to the active pharmaceutical ingredient(s) to assist in the manufacturing process as well as to optimise drug delivery. Due to advances in drug delivery technology, excipients are currently included in novel dosage forms to fulfil specific functions and in some cases they directly or indirectly influence the extent and/or rate of drug release and absorption. Since plant polysaccharides comply with many requirements expected of pharmaceutical excipients such as non-toxicity, stability, availability and renewability they are extensively investigated for use in the development of solid oral dosage forms. Furthermore, polysaccharides with varying physicochemical properties can be extracted from plants at relatively low cost and can be chemically modified to suit specific needs. As an example, many polysaccharide-rich plant materials are successfully used as matrix formers in modified release dosage forms. Some natural polysaccharides have even shown environmental-responsive gelation characteristics with the potential to control drug release according to specific therapeutic needs. This review discusses some of the most important plant-derived polymeric compounds that are used or investigated as excipients in drug delivery systems.

Site specific delivery of microencapsulated fish oil to the gastrointestinal tract of the rat

The aim of this study was to design food grade matrices to deliver microencapsulated fish oil to the large bowel of the rat where the potential exists to retard inflammation and cancer development. Digestion in simulated gastric fluid and intestinal fluid demonstrated that only 4-6% of oil was released from the following dried emulsion formulations: 50% fish oil encapsulated in heated casein-glucose-dried glucose syrup (1:1:1) (Cas-Glu-DGS-50); 25% fish oil in casein-modified resistant starch (Hylon VII) (1:1) (Cas-Hylon-25); or 25% fish oil in Cas-Glu-Hylon (1:1:1) (Cas-Glu-Hylon-25). A short-term gavage study (0-12 h) with fish oil and Cas-Glu-DGS-50 demonstrated the appearance of fish oil long chain (LC) n-3 polyunsaturated fatty acids (PUFA) into the plasma indicating specific small intestinal absorption with little LC n-3 PUFA reaching the large bowel. In a 2-week-long term, daily gavage study, the bioavailability of fish oil and fish oil in Cas-Glu-DGS-50 or Cas-Hylon-25 demonstrated that fish oil and Cas-Glu-DGS-50 LC n-3 PUFA were incorporated into the tissue of the small intestine and colon, whereas Cas-Hylon-25 was resistant to degradation in the small intestine. The use of modified Hylon VII for targeted colonic delivery was confirmed in the final short-term gavage study (0-14 h) using Cas-Glu-Hylon-25 with [(14)C]-trilinolenin as a marker incorporated into the microcapsules, where up to 60% of the labeled oil reached the large bowel. Depending on the microencapsulating matrix employed, fish oil can be delivered selectively to the small intestine or to a high degree to the large bowel.

Carboxylated high amylose starch as pharmaceutical excipients. Structural insights and formulation of pancreatic enzymes

Carboxymethyl high amylose starch (CM-HAS) and succinate high amylose starch (S-HAS) were proposed as pharmaceutical excipients for oral drug delivery, providing a significant gastroprotection to dosage forms of pancreatic enzymes (alpha-amylase, lipase and trypsin) compared to unprotected enzymes. In acidic medium, carboxylic groups are protonated (at least in tablet surface) ensuring local buffering properties and giving a compact shape of the tablets. The enzymes were formulated individually or in association as three enzymes formulation. After the first hour of incubation (over a 2h experiment) in simulated gastric fluid (SGF), the three pancreatic enzymes retained an overall (average of the three enzymes) activity of 72% when formulated as tablets with CM-HAS excipient and 77% when formulated with S-HAS excipient. Furthermore, after incubation in SGF, the delivery of 75% of the total remaining enzymatic activity in the simulated intestinal fluid (SIF) taken 180 and 170 min for CM-HAS and S-HAS, respectively. Both formulations with carboxylated starch as excipient have a high loading capacity (up to 70-80% enzymes), which is of interest for pancreatic enzymes replacement therapy of pancreatitis. An advantage of these formulations is that gastroprotection is afforded by the carboxylated matrices (carboxylic groups), without enteric coating.

Study of hydration of cross-linked high amylose starch by solid state 13C NMR spectroscopy

Starch is subjected to chemical treatments such as cross-linking or hydroxypropylation to meet the material requirements for food uses or controlled release in the pharmaceutical industries. In this work, two types of cross-linking formulations have been employed for the preparation of high amylose starch for use as an excipient for sustained drug release. The structural differences and chain dynamics of the modified starches in the dry and hydrated states have been compared by the use of variable contact time cross polarization-magic angle spinning solid state (13)C NMR spectroscopy.

Cross-linked starch microspheres: Effect of cross-linking condition on the microsphere characteristics

Cross-linked starch microspheres were prepared using different kinds of cross-linking agents. The influence of several parameters on morphology, size, swelling ratio and drug release rate from these microspheres were evaluated. These parameters included cross-linker type, concentration and the duration of cross-linking reaction. Microspheres cross-linked with glutaraldehyde had smooth surface compared with those prepared with epichlorhydrine or formaldehyde. The particle size increased with increasing the cross-linking time and increasing the drug loading. Swelling ratio of the particles was a function of cross-linker type but not the concentration or time of cross-linking. Drug release from starch microspheres was measured in phosphate buffer and also in phosphate buffer containing alpha-amylase. Results showed that microspheres cross-linked with epichlorhydrine released all their drug content in the first 30 minutes. However, cross-linking of the starch microspheres with glutaraldehyde or formaldehyde decreased drug release rate. SEM and drug release studies showed that cross-linked starch microspheres were susceptible to the enzymatic degradation under the influence of alpha-amylase. Changing the enzyme concentration from 5000 to 10,000 IU/L, increased drug release rate but higher concentration of enzyme (20,000 IU/L) caused no more acceleration.

A novel method for the preparation of biodegradable microspheres for protein drug delivery

Microspheres are potential candidates for the protein drug delivery. In this work, we prepared polymer-coated starch/bovine serum albumin (BSA) microspheres using co-axial electrohydrodynamic atomization (CEHDA). First, starch solution in dimethyl sulphoxide (DMSO) was prepared and then an aqueous solution of BSA was added to it to make a starch-BSA solution. Subsequently, this solution was made to flow through the inner capillary, while the polymer, polydimethylsiloxane (PDMS), flowed through the outer capillary. On collection, filtration and subsequent drying, near-monodisperse microspheres of 5-6microm in size were obtained. The microspheres were characterized by Fourier-transform infrared (FT-IR) spectroscopy and scanning electron microscopy. Cumulative BSA release was investigated by UV spectroscopy. BSA structure and activity was preserved in the microspheres and its release in 0.01M phosphate buffered saline (PBS) was studied over a period of 8 days. There was an initial burst with 32wt% of total BSA released in 2h. Overall 75wt% of BSA was released over a 7 day period.

Imaging of High-Amylose Starch Tablets. 3. Initial Diffusion and Temperature Effects

The penetration of water into cross-linked high amylose starch tablets was studied at different temperatures by nuclear magnetic resonance (NMR) imaging, which follows the changes occurring at the surface and inside the starch tablets during swelling. It was found that the swelling was anisotropic, whereas water diffusion was almost isotropic. The water proton image profiles at the initial stage of water penetration were used to calculate the initial diffusion coefficient. The swelling and water concentration gradients in this controlled release system show significant temperature dependence. Diffusion behavior changed from Fickian to Case II diffusion with increasing temperature. The observed phenomena are attributed to the gelatinization of starch and the pseudo-cross-linking effect of double helix formation.

The influence of excipients on drug release from hydroxypropyl methylcellulose matrices

The influence of commonly used excipients, spray-dried lactose (SDL), microcrystalline cellulose (MCC), and partially pregelatinized maize starch (Starch 1500) on drug release from hydroxypropyl methylcellulose (HPMC, hypromellose) matrix system has been investigated. A model formulation contained 30%w/w drug, 20%w/w HPMC, 0.5%w/w fumed silica, 0.25%w/w magnesium stearate, and 49.25%w/w filler. Chlorpheniramine maleate and theophylline were used as freely (1 in 4) and slightly (1 in 120) water-soluble drugs, respectively. It was found that for both drugs, addition of 20 to 49.25%w/w Starch 1500 resulted in a significant reduction in drug release rates compared to when MCC or SDL was used. The study showed that using lactose or microcrystalline cellulose in the formulations resulted in faster drug release profiles. Partially pregelatinized maize starch contributed to retardation of both soluble and slightly soluble drugs. This effect may be imparted through synergistic interactions between Starch 1500 and HPMC and the filler actively forming an integral part within the HPMC gel structure.