Use of Dicalcium Phosphate Dihydrate for Sustained Release of Highly Water Soluble Drugs

The utilituy of Dicalcium phosphate dihydrate (DCPD), a commonly used water insoluble pharmaceutical excipient, has been investigated for its use in formulating a controlled release matrix type tablet for highly water soluble drugs. Various drugs were formulated into a tablet by direcly compressing mixture of the drug, dicalcium phosphate dihydrate, and magnecium stearate on a single punch tablet machine. Effects of drug concentation and tablet weight on the release profiles were studied using USP II dissolution machine.

The release from these matrices followed first order kinetics rather than square root time rule. The release profile and the first order rate constant seemed to be dependent upon the the size of the tablet. Incorporation of drug in a quantity excess of 5% w/w of the tablet resulted in disintegration of the tablet and subsequent rapid release of the drug.

Dicalcium phosphate may be a simpler, cheaper, and a viable way to formulating directly compressible sustained release formulations.     

Matrix Type Tablet Formulation for Controlled Release of Highly Water Soluble Drugs

Matrix type formulations with dicalcium phosphate dihydrate (DCPD) using a polymeric binder (Eudragit RSPM®) to obtain controlled release of highly water soluble drugs has been investigated.

The drug, DCPD and Eudragit RSPM® were granulated using isopropyl alcohol with and without a plasticizer (Diethyl phthalate, DEP). Addition of Eudragit did not appear to affect the release profile. However, addition of a plasticizer had a significant effect on the rate of release. The release appears to follow first order kinetics and the rate constant decreased linearly with increasing DEP concentration.

A directly compressible mixture was also formulated by coating DCPD particles with DEP with and without Eudragit RSPM®.     

Studies on Drug Release from a Carbomer Tablet Matrix

The purpose of this investigation was to study the drug release mechanisms for tablet matrices of carbomer. Carbomer is a polymer of acrylic acid which is cross-linked with polyalkenyl polyether. The drug and the carbomer were blended and directly compressed into tablets using a laboratory Carver press. The influence of the level of carbomer, the type of drug, and the pH of dissolution media were investigated by measuring drug release kinetics. In general, the release of a relatively neutral molecule (e.g. theophylline) in the pH 7.2 phosphate buffer solution appears to exhibit nearly zero-order kinetics via a diffusion-controlled mechanism for all polymer levels studied (10-85%).

The drug release process based on diffusion can be described by the general expression:

M_t = k₁t^1/2 + k₂t

where M, represents the amount of the drug released at time t, and k₁, k₂ are related to kinetic constants characteristic of the drug delivery systems. The release kinetics are modified when an ionic species, such as sodium salicylate, is incorporated into the tablet matrix.     

Compatibility Study Between Atenolol and Tablet Excipients Using Differential Scanning Calorimetry

In order to improve the formulation of atenolol the physico-chemical compatibility between the drug and various excipients, commonly used in tablet manufacturing, was studied with the aid of Differential Scanning Calorimetry (DSC).

Using this method, it was found that atenolol is compatible with starch, Sta-Rx®, Primojel®, Avicel PH®, Ac-Di-Sol®, cross-linked PVP, magnesium stearate, calcium sulphate dihydrate, dicalcium phosphate and icing sugar. Interactions of atenolol with PVP, lactose and the lubricant stearic acid were found, although it cannot be conlusively stated that interaction incompatibilities will occur during storage at room temperature.     

A Simple Model Based on First Order Kinetics to Explain Release of Highly Water Soluble Drugs from Porous Dicalcium Phosphate Dihydrate Matrices

A simple model was developed to explain release of highly water soluble drugs from inert, insoluble, nonswelling porous matrices. According to this model the release can be explained using a first order kinetic expression: Q = Q_{o e}-Kt, where Q is amount released, Q_o is initial amount, and K is rate constant. The rate constant is related to the geometry of the matrix as: K = K_b A/V where, K_b is a diffusion related proportionality constant, A is void area, and V is void volume. For cylindrical matrices, the rate constant can be expressed as K = K_b 2(1/r + 1/h) where r is radius and h is height of the matrix.

Cylindrical as well as biconvex matrices were prepared on a single punch tablet machine with varying heights and radii, thus different specific surface areas. The rate constants were determined following dissolution testing. The experimental release profiles follow first order kinetics. Good correlation was found between the rate constant and specific surface area of the matrices studied.     

Hydroxypropylmethylcellulose sustained release technology

The use of polymers in controlling the release of drugs has become important in the formulation of pharmaceuticals. Watersoluble polymers such as polyethylene glycol and polyvinylpyrrolidone may be used to increase the dissolution rates of poorly soluble drugs (Ford)¹ and slowly soluble, biodegradable polymers such as polylactic acid may be used for controlled release implants (Rak et a1.²), Hydrogels provide the basis for implantation, transdermal and oral-controlled release systems. Hydroxypropylmethylcellulose (HPMC) are cellulose ethers which may be used as the basic for hydrophilic matrices for controlled release oral delivery.

In tablet matrix systems the tablet is in the form of compressed compact containing an active ingredient, lubricant, excipient, filler or binder. The matrix may be tabletted from wet-massed granules or by direct compression.

This review article examines a previously published series of work and concentrates on the following aspects of the subject; the relationship between release rate and quantity of polymers, such consideration allow a certain predicability in release rates to be made. Also the effect of drug particle size, tablet shape and the presence of additional diluents in the formula are examined.     

Investigation of Prolonged Drug Release from Matrix Formulations of Chitosan

A hydrocolloidal matrix system containing complexes of chitosan was investigated for preparation of sustained release tablets and examined in-vitro.

Theophylline tablets using chitosan as a sustained release base were evaluated. It was found that when chitosan is used in a concentration of more than 50% of tablet weight, an insoluble non-erosion type matrix was formed. Tablets prepared with a chitosan concentration of less than 33% were fast releasing.

Chitosan used in a concentration of about 10% acted as a disintegrant and the drug was dissolved within an hour.

Citric acid slowed down the release rates of chitosan based theophylline tablets. Theophylline tablets using carbomer-934P as a sustained release base were evaluated. Carbomer-934P in lower concentrations forms an erosion type matrix. In order to produce a twenty-four (24) hour sustained release tablet, more than 10% concentration of carbomer-934P is needed. Combination with chitosan and carbomer-934P produced slower releasing tablets.

A hydrocolloidal erosion type matrix was formulated using chitosan, carbomer-934Pand citric acid. Only 10% of chitosan was needed to prepare theophylline sustained release tablets in these mixtures.

The dose dumping potential of chitosan tablets due to rapid disintegration in alkaline media was eliminated by preparing hydrated erosion type matrix systems.     

Release of cyclobenzaprine hydrochloride from osmotically rupturable tablets

Osmotically rupturable systems were developed and the release of cyclobenzaprine hydrochloride (model drug) from the systems was investigated. Systems were designed using mannitol (osmotic agent) and increasing amounts of polyethylene oxide (PEO, a water-swellable polymer) surrounded by a semipermeable membrane. When placed in an aqueous environment, osmotic water imbibition into the systems distended and swelled the systems until the membrane ruptured and released the active compound to the outside environment. Tablets with increasing amount of PEO exhibited longer rupture times. This may be due to osmotic pressure-modulating properties of the polymer, changing the rate of water imbibition into the systems.

The integrity of the membranes was investigated using high-pressure mercury intrusion porosimetry. Minimal mercury intrusion into the membrane structure and core tablet indicated membrane integrity and lack of defective areas or pinholes. The results were in agreement with the release profiles where no drug release was detected prior to membrane rupture. Mercury intrusion porosimetry appears to be a promising technique for evaluation of membrane integrity.

Once the systems ruptured, drug was released by osmotic pumping and diffusion mechanisms through the ruptured area. There was a decrease in drug release rate with inclusion of PEO in the core.

The effects of film thickness on rupture and release times were also investigated. Devices with thicker films produced longer rupture times. This is in agreement with the theoretical prediction.     

Design and evaluation in vitro of controlled release mucoadhesive tablets containing chlorhexidine

This investigation deals with the development of buccal tablets containing chlorhexidine (CHX), a bis-bis-guanide with antimicrobial and antiseptic effects in the oral cavity, and able to adhere to the buccal mucosa to give local controlled release of drug. A mucoadhesive formulation was designed to swell and form a gel adhering to the mucosa and controlling the drug release into the oral cavity.

Some batches of tablets were developed by direct compression, containing different amounts of hydroxypropylmethylcellulose (HPMC) and carbomer; changing the amount ratio of these excipients in formulations, it is possible easily modulate the mucoadhesive effect and release of drug. The in vitro tests were performed using the USP 26/NF paddle apparatus, a specifically developed apparatus, and a modified Franz diffusion cells apparatus. This last method allows a simultaneous study of drug release rate from the tablets and drug permeation through the buccal mucosa.

Similar tests have also been carried out on a commercial product, Corsodyl gel^®, in order to compare the drug release control of gel with respect to that of the mucoadhesive tablet, as a formulation for buccal delivery of CHX. While the commercial formulation does not appear to control the release, the formulation containing 15% w/w methocel behaves the best, ensuring the most rapid and complete release of the drug, together with a negligible absorption of the active agent as required for a local antiseptic action in the oral cavity.     

Carbamazepine Modified Release Dosage Forms

The preparation of sustained release dosage forms of Carbamazepine (anti-epileptic drug characterized by a very low water solubility and by a short half life on chronique dosing) was carried out.

These formulations were obtained in two different steps:

a) modified release granules were prepared by the loading of cross-linked sodium carboxymethylcellulose (swellable polymer), with the drug and an enteric polymer. Cellulose acetate phthalate, methacrylic acid - methacrylic acid methyl ester copolymer (usually employed as enteric coating agents) and cellulose acetate trimellitate (a new enteric polymer) were used in different weight ratios.

b) some sustained release dosage forms were prepared tabletting physical mixtures of the modified release granules with different weight ratios of hydroxypropylmethylcellulose.

In vitro dissolution tests of modified release granules in gastric fluid (USP XXI) showed a modulation of the drug release, while in intestinal fluid (USP XXI) a quick drug dissolution was observed.

In vitro dissolution tests of sustained release dosage forms, performed varying during the test, the pH of the dissolution medium, (hydrochloric acid pH 1 from 0 to 2 hours and phosphate buffer pH 6.8 from 2 to 18 hours) showed that the determining factors in the controlling release of the drug are: the type and amount of enteric polymer constituting the granules and the amount of hydroxy-propylmethylcellulose mixed with them.     

Decomposition and Stabilization of the Tablet Excipient Calcium Hydrogenphosphate Dihydrate

Calcium hydrogenphosphate dihydrate is a commonly used filler in solid dosage forms. In the literature it is noticed that drug decomposition reactions can be accelerated by the decomposition of CaHPO₄.2H₂O. The present study has been performed to evaluate various types of this commercially available excipient and to elucidate the impact of selected factors on stability.

The influence of both temperature (40, 50 and 60°C) and environmental humidity (26, 34 and 46 % rel. humidity) on the dehydration of calcium hydrogenphosphate dihydrate was studied for a period of two to three months. Five batches of CaHPO₄.2H₂O were used with different contents of magnesium hydrogenphosphate and sodium pyrophosphate, which are often added to stabilize CaHPO₄.2H₂O in preparations like aqueous dentifrices. Disks of CaHPO₄.2H₂O have been prepared in such a way, that the structure of the disks allowed discrimination between the batches in dehydration kinetics. A relatively low velocity of dehydration was found for the batches containing high levels of sodium pyrophosphate. The effect of environmental humidity on the dehydration velocity appeared not to be straightforward: only at 60 °C an acceleration of the dehydration rate was found for all batches due to the increase of relative humidity. The residues after dehydration were analysed by Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT), Thermogravimetric Analysis and Differential Scanning Calorimetry.

The results demonstrate that, independent of the storage condition and the type of calcium hydrogenphosphate dihydrate used, the main decomposition product is anhydrous calcium hydrogenphosphate. This contrasts with interpretations in the literature, which include the formation of decomposition products like hydroxyapatite and phosphoric acid.

Although no experiments were performed in the presence of drugs, a hypothesis is proposed on the basis of this study for the mechanism of possible drug decomposition in the presence of calcium hydrogenphosphate dihydrate. The mechanism can be simplified to dissolution of drug in accumulated free water due to dehydration within the drug/excipient system with, possibly, interactions between the drug and calcium hydrogenphosphate dihydrate. Obviously, time, temperature and amount of accumulated free water are important factors in determining the amount of drug decomposed. Accumulation of free water in granulates and tablets containing calcium hydrogenphosphate dihydrate should therefore be avoided or at least limited. This will have implications for e.g. the selection of the quality of calcium hydrogenphosphate dihydrate to be used and for the manufacturing practice of solid dosage forms containing this filler excipient.     

Development of a Trans-Mucosal Controlled-Release Device for Systemic Delivery of Antianginal Drugs Pharmacokinetics and Pharmacodynamics

Oral mucosa is well-known to be one of the best routes for drug absorption. But very few R & D works have been initiated to investigate the feasibility of using this site to control drug delivery. A transmucosal controlled-release device, which is capable of achieving excellent absorption and controlled release of drugs, has been developed. The device is a tabletshaped mucoadhesive system which is composed of two layers. The upper layer is a fast-release layer and the lower layer is a sustained-release layer, and designed to be applied between buccal and gingival mucosae. Both layers are formulated from synthetic polymers to control the release of drugs.

Isosorbide dinitrate(ISDN), a well-documented antianginal drug, is known to be susceptible to extensive presystemic elimination when taken orally. It was used as the candidate drug and the systemic bioavailability was studied in human and observed to be improved by as much as 5 fold when compared to a marketed oral sustained-release tablet; On the other hand, much smaller amount of metabolites was formed. The plasma profile of ISDN has also been observed to be substantially prolonged (12 hrs as compared to less than 1 hr for sublingual tablet and spray product on the market). These observations have demonstrated that this device is capable of not only bypassing hepatic “first-pass” metabolism but also having a sustainedrelease property of prolonging the release of ISDN.

Clinical studies performed in the anginal patients for up to one year have demonstrated the therapeutic benefits of this device in achieving a substantial reduction in the frequency of anginal attacks.

This type of device was also applied to the systemic delivery of another antianginal drug, Nifedipine, by employing a formulation with longer sustained drug release property. Again, the clinical results demonstrated that a prolonged duration of therapeutic plasma concentration has also been accomplished.     

Solid Dispersion Controlled Release: Effect of Particle Size, Compression Force and Temperature

Solid dispersions are dynamic systems, a careful control of processing variables is required to produce desired physicochemical properties of these systems.

The influence of drug particle size, dispersion temperature and compression force on the release rate of theophylline from solid dispersed system tablets was studied. Theophylline base (micronized and granulate) were embedded into a polymeric mixture of PEG and acrylic/methacrylic esters at controlled temperature and shock cooled. Tablets were made at two compressional forces and drug release was measured spectrophotometrically over a period of fifteen hours.

The release rate of drug dispersed in these insoluble matrices was independent of particle size but not of hardness.

However, variations in ratios of polymeric mixture and dispersion temperature controls the drug release rate from inert matrix more effectively than such factors as drug particle size and lower range of tablet hardness. The fast cooling produced excellent reproducibility of drug content throughout the entire entrapment product. X-ray diffraction study demonstrated no changes in crystalline form of theophylline.     

Technological Evaluation of three Enteric Coating Polymers I. With an Insoluble Drug

The enteric properties of a recent cellulose polymer, cellulose acetate trimellitate (CAT, EASTMAN KODAK) were evaluated on an insoluble substract for comparison, included in this paper are the properties of two other cellulose esters: cellulose acetate phthalate (CAP) and hydroxypropyl methylcellulose phthalate (HP55).

The physical properties and disintegration time at pH 1.2 and 6.5 were influenced by the level of coating solution. The gastroresistance was obtained more fastly with CAT and CAP than for HP55.

The influence of coating solution on drug release from tablet was investigated. The dissolution studies were made allowing the variation of pH in the dissolution medium during the kinetics.

Drug release from coated tablets was found to be dependent upon the type of polymers used to form film: higher release rates were obtained with CAT compared to CAP and HP55.     

Selection of Optimum Dissolution Test Methods in Dosage Form Design

The rate limiting factors involved in release of a drug from a tablet are generally accepted to be the disintegration of the tablet followed by the subsequent dissolution of the drug from the dispersed granules.

The development of new potencies of an existing tabletted product by weight multiplication was found to result in non-conformance to established specifications for the product not formulation related.

In addition to a study of the rate limiting factors mentioned above compendial dissolution tests were also compared. Apparatus II was shown to produce more rapid and consistent results than Apparatus I in this investigation and this test is recommended as the one of choice where large volume compacts are involved.     

In-vitro evaluation of sustained-release dyphylline tablets

Dyphylline tablets were prepared by direct compression of mixtures of the drug, emcompress and different ratios of hydroxypropyl methylcellulose (HPMC) or cellulose acetate phthalate (CAP). Physical properties of the prepared tablets and the drug release in 0.1 N HC1 and phosphate buffer, pH 7.4 were investigated.

All tablets were found to satisfy the USP requirements regarding content, weight uniformity and friability. Hardness was greatly enhanced and thickness was slightly increased by increasing the polymer ratio in tablet formulations. Disintegration time of the dyphylline tablets was delayed by the presence of either HPMC or CAP and there was a direct relationship between the polymer ratio and the disintegration time. Considerable retardation in the rate and extent of drug release from the prepared tablets in both dissolution liquids was observed. As the polymer ratio increased in the tablet formulations, the drug release was significantly inhibited.     

Disintegrants in Solid Dosage Forms

The dynamic approach to tablet disintegration, which is based on the measurement of the force that develops inside the compact upon water entrance, is basically taken up.

The combined measurements of force development and water uptake, simultaneously effected on the same compact, provide a novel parameter that is proposed to quantify and compare the efficiency of disintegrants.

The new parameter, which is based on the “force-equivalent” concept, expresses the capability of a disintegrant of transforming water uptaken into swelling (or disintegrating) force. A few examples, that illustrate the usefulness of this parameter for disintegrant characterization, are given.

In parallel to the quantification of swelling (or disintegrating) efficiency inside compacts, attention is also being paid to the characterization of swelling disintegrants as pure materials.

In particular the case of the so-called limited swelling materials, for which the quantification of intrinsic swelling (particle volume increase in swelling media) is critical, is considered.

The applicability of an instrumental method, which is based on the employment of a Coulter Counter, is discussed alternatively to microscopic methods.

Disintegrant characterization may also be considered in view of new possible exploitations of the swelling properties of polymers in controlling drug release.     

Evaluation of the Granulation of a Hydrophilic Matrix Sustained Release Tablet

Wet granulation of a hydrophilic sustained release matrix tablet formulation has been studied. A fractional factorial experimental design was employed to identify principal influences and interacting factors from the following : granulation fluid volume, mixing time, mixer speed and inclusion of a wet screening step. Fluid volume and mixing time were primary factors affecting mean granule size. Fines in the granulation were reduced at higher fluid levels and by inclusion of a wet screening operation. There were several interacting factors influencing the particle size properties of the granulation. The factors studied had little influence on the bulk density of the granulation.

The influence of granule mean particle size on flow, compressibility and drug release from finished tablets was evaluated. Flow and compressibility were influenced by granule properties and the data generated suggested that should final tablet properties deteriorate on scale up it may be possible to ameliorate the effect by modification of granulation fluid volume or mixing time or both.

The factors studies had no influence on release of drug from finished tablets.     

Disintegration Mechanisms of Tablets Containing Starches. Hypothesis About the Particle-Particle Repulsive Force

The disintegration of a tablet immersed in a liquid appears to be essentially a mechanical phenomenon: penetration of liquid then destruction of compressed structure

For a number of authors, the starch granule swelling is the mechanical force which destroys the tablet.

Indeed, a study on a series of experimental carboxymethylstraches indicates that those that do not swell, show the same disintegration time to those that do swell.

We can also notice that the carboxymethylstarches wich swell much less in a gastric medium, produce even shorter disintegration times in this medium.

The destruction of the cohesion forces between the constitutive elements of the tablet under the action of water may be ascribed to the creation of a repulsive force when the elements of the tablet enter into contact with water, or to a simple annihilation of the hydrogen bonds or of the capillary cohesion forces.

The hydrophilic nature of starch seems to be determinant: water penetrates into the tablet owing to hydrophilic porosity under the action of an important hydrostatic pressure.     

Miconazole and Miconazolenitrate Chewing Gun as Drug Delivery Systems - A Practical Application of Solid Dispersion Technique

Miconazole and miconazolenitrate are antifungal drugs with poor solubilities in water and saliva. The low solubilities meant that only small amounts of the drugs - incorporated by a conventional method in chewing gum-were released during mastication. The experiments were performed on a mastication device.

In this study it was shown that application of a 20% miconazole - 80% polyethyleneglycol 6000 solid dispersion drastically improved the in vitro release of miconazole from cheving gum, when a medium similar to saliva was used. In addition to polyethyleneglycol 6000, polyvinylpyrrolidone 40000, xylitol and urea were tested as carriers. It was also shown that the release rate of miconazole from chewing gum was much greater than the release rate of miconazolenitrate.

No certain correlation could be shown between the dissolution rates of the solid dispersions measured by a stirring paddle method and the release rates of miconazole from solid dispersions in chewing gum.

The solid dispersion systems were characterized by differential scanning calorimetry. The systems containing polyethyleneglycol 6000 and xylitol were eutectic. Polyvinylpyrrolidone 40000 prevented crystallisation of miconazole when the percentage of drug in the solid dispersion was less than 50%.