Characterisation of water behaviour in cellulose ether polymers using low frequency dielectric spectroscopy

The behaviour of water in hydroxypropylmethylcellulose (HPMC) K100LV, K4M, K15M, K100M, E4M, F4M and HPC polymers was characterised using low frequency dielectric spectroscopy (LFDS). Dielectric responses of 25% (w/w) HPMC K15M gels and deionised water were found to be similar at +22 and 0 degrees C. However, at -30 degrees C, a dielectric response typical of a solid was apparent. The melting of frozen water within gels was detected as increases in the magnitude of the dielectric response with increase in temperature. More than one phase transition was visible in the majority of gels studied which may be related to the presence of different states of water melting at different temperatures. In addition to polymer concentration, both polymer molecular weight and substitution level influenced the nature of the transitions. The magnitude of the dielectric response was increased in all HPMC gel systems in comparison to the response seen in deionised water. Drug addition affected the transitions occurring during the melting of ice in the gels. This may be related to the presence of ionic species in the systems. LFDS studies on cellulose ether gels have provided some interesting evidence for the existence of more than one state of water within such gel systems. The results are in good agreement with thermal analysis findings in similar gel systems.

Effect of compression force, compression speed, and particle size on the compression properties of paracetamol

The compression characteristics of two particle size fractions (< 90 microm, 105-210 microm) of paracetamol were examined. Each fraction produced extremely weak tablets and displayed a high tendency to cap. Low correlation coefficients of the initial parts of the Heckel plots, a low strain rate sensitivity, and an increase in mean yield pressure (from 34.2 to 45.5 MPa) with decrease in particle size all confirmed that the main mechanism during the compaction of paracetamol was fragmentation. The 105-210-microm particles underwent more fragmentation than the less than 90-microm powder. Heckel analysis confirmed that the larger size fraction of paracetamol produced denser compacts than the smaller fraction. The 105-210-microm fraction resulted in tablets with lower elastic recoveries and elastic energies. The elastic, plastic energy ratios indicated that the majority of energy involved during the compaction of paracetamol was utilized as elastic energy, indicative of massive elastic deformation of paracetamol particles under pressure.

Crystal habit modifications of ibuprofen and their physicomechanical characteristics

Ibuprofen was crystallized from methanol, ethanol, isopropanol, and hexane at similar conditions. Marked differences in crystal habit of the samples obtained from these solvents were observed. The samples crystallized from methanol and ethanol had a polyhedral crystal habit, while those from hexane were needlelike. Those from isopropanol were elongated crystals. X-ray powder diffraction (XPD) and diferential scanning calorimetry (DSC) studies confirmed that these samples were structurally similar, therefore, polymorphic modifications were ruled out. The results showed that crystal habit modification had a great influence on the mechanical properties (compressibility, flow rate, and bulk density) of ibuprofen crystals. Samples obtained from methanol and ethanol exhibited the highest bulk density and the best flow rate, while those from hexane showed the lowest bulk density and the worst flow rate. The samples obtained from ethanol exhibited the best compression force/hardness profiles, and those obtained from hexane produced the softest tablets.

Study of drug release from pellets coated with Surelease containing hydroxypropylmethylcellulose

The release of metoclopramide hydrochloride (a very water soluble cationic drug) and diclofenac sodium (a sparingly soluble anionic drug) from pellets coated with Surelease containing hydroxypropylmethylcellulose (HPMC) at different coating loads was investigated. The release rates of either drug at each coating composition decreased as the coating load increased. Inclusion of HPMC E15 increased the release rates of both drugs compared to pellets coated only with Surelease. This was thought to be due to the leakage of the soluble part of the film (HPMC E15) during dissolution, which left pores for drug release. The Surelease:HPMC E15 ratio had a major role in the release rates of drugs. Addition of HPMC E15 into Surelease did not change the release mechanism for metoclopramide hydrochloride (the mean value of n approximately 0.57) from that of Surelease alone, and diffusion remained the main mechanism controlling the release. However, the release exponent (approximately 1.28) increased for diclofenac sodium on addition of HPMC E15, indicating a dissolution-controlled mechanism. Despite its lower water solubility, diclofenac sodium was released slightly faster than metoclopramide hydrochloride from pellets coated with Surelease containing HPMC E15 at equivalent coating loads.

Highly compressible paracetamol: I: crystallization and characterization

It was found that polyvinylpyrrolidone (PVP) is an effective additive during crystallization of paracetamol and significantly influenced the crystallization and crystal habit of paracetamol. These effects were attributed to adsorption of PVP onto the surfaces of growing crystals. It was found that the higher molecular weights of PVP (PVP 10000 and PVP 50000) were more effective additives than lower molecular weight PVP (PVP 2000). Paracetamol particles obtained in the presence of 0.5% w/v of PVP 10000 or PVP 50000 had near spherical structure and consisted of numerous rod-shaped microcrystals which had agglomerated together. Particles obtained in the presence of PVP 2000 consisted of fewer microcrystals. Differential scanning calorimetry (DSC) and X-ray powder diffraction (XPD) experiments showed that paracetamol particles, crystallized in the presence of PVP, did not undergo structural modifications. By increasing the molecular weight and/or the concentration of PVP in the crystallization medium the amount of PVP incorporated into the paracetamol particles increased. The maximum amount of PVP in the particles was 4.32% w/w.

Highly compressible paracetamol - II. Compression properties

Paracetamol particles crystallized in the presence of polyvinylpyrrolidone (PVP) exhibited an obvious improvement in their compression properties compared to untreated paracetamol. Paracetamol particles crystallized in the presence of 0.5% w/v PVP 10000 or PVP 50000 produced tablets with improved crushing strength with no tendency to cap even at high compression speeds. The very low values of strain rate sensitivity (SRS) and the lack of reduction in crushing strength with increasing compression speed for these particles, were indicative of a high degree of fragmentation during compression. The results of elastic recoveries and elastic energies of tablets were indicative of much less elastic behaviour of these particles than untreated paracetamol. The low elastic energy/plastic energy (EE/PE) ratio for paracetamol crystallized in the presence of PVP indicated that, contrary to untreated paracetamol, a minor portion of compression energy was utilized as elastic energy.

Comparative study of drug release from pellets coated with HPMC or Surelease

The release of metoclopramide hydrochloride (very water soluble cationic drug) and diclofenac sodium (sparingly soluble anionic drug) from pellets coated with hydroxypropylmethylcellulose (HPMC; water-soluble polymer) or ethylcellulose aqueous dispersion (Surelease; water-insoluble polymer) at different coating loads was investigated. The release rates of either drug decreased as the coating load of HPMC increased, but overall, the release was fast, and the majority of both drugs released in about 1 hr, even at the highest coating load. The drug release mechanism for either drug was not affected by the coating load of HPMC or by the type of drug used, and it was found to be mainly diffusion controlled. Diclofenac sodium released slightly more slowly than metoclopramide hydrochloride from HPMC-coated pellets. This was attributed to the lower water solubility of the former drug. The release rate of either drug decreased greatly as the coating load of Surelease increased. The release of both drugs was sustained over 12 hr as the coating load of Surelease increased, and only about 70% of either drug was released after this period at the highest coating load (20%). The mechanism of release of metoclopramide hydrochloride was independent of coating load, and it was predominantly diffusion controlled. However, the mechanism of diclofenac sodium release was dependent on the coating load of Surelease. At low coating loads, diffusion of drug was facilitated due to the presence of more pores at the surface of the coated pellets; therefore, the rate of dissolution of the drug particles was the rate-limiting step. However, at high coating loads, drug release was mainly diffusion controlled. Despite its lower water solubility, diclofenac sodium released slightly faster than metoclopramide hydrochloride from Surelease-coated pellets at equivalent coating loads.

Principles and application of ultrasound in pharmaceutical powder compression

The use of ultrasound during the tableting of pharmaceutical powders is a new concept. However, in the metallurgy, plastic, and ceramic industries, ultrasound-assisted compression of materials has been known for some years. Ultrasound improves the characteristics of the compression process leading to optimized mechanical strength of the compacts without applying excessive compression force. Therefore, problems associated with high-pressure compression in tableting can be overcome and tablets may be manufactured more economically and consistently with the aid of ultrasound compared to conventional pressure processes. Although great progress in the theoretical understanding of the ultrasound-assisted powder compression process has been made since the late 1960s, the need for further research in the area of ultrasound application during pharmaceutical powder compression is essential. Further investigations on a wide range of drugs and excipients, to expand the usefulness and scope of the ultrasound-assisted technique, and to understand the complex phenomena involved in the process, are needed. In this article the principles, advantages, and limitations of the application of ultrasonic vibrations during pharmaceutical powder compression is reviewed with the hope that this article can contribute to, and stimulate research in the area.

Formation and compression characteristics of prismatic polyhedral and thin plate-like crystals of paracetamol

Prismatic polyhedral crystals of paracetamol were prepared by cooling an aqueous saturated solution of paracetamol from 65 to 25 degrees C. Thin plate-like crystals were prepared by adding a concentrated solution of paracetamol in hot ethanol to water at 3 degrees C. Infrared (IR), X-ray powder diffraction (XPD) and differential scanning calorimetry (DSC) studies confirmed that these two forms of crystals were structurally similar, therefore polymorphic modifications were ruled out. The crystal habit influenced the compression properties during axial compression of paracetamol at different constant rates in a compaction simulator, the Heckel plots and their associated constants being dependent on the habits. The correlation coefficient of the initial part of the Heckel plots, and also the values of strain rate sensitivity (SRS), were lower for thin plate-like crystals, indicative of greater fragmentation for the thin plate-like as compared to polyhedral crystals. Compacts made from thin plate-like crystals exhibited higher elastic recoveries and elastic energies indicating that these crystals underwent less plastic deformation during compression than the polyhedral crystals.

Determination of the glass transition temperatures of some new methyl methacrylate copolymers using modulated temperature differential scanning calorimetry (MTDSC)

PURPOSE

The purpose of this study was to determine the glass transition temperatures of new graft copolymers using Modulated Temperature Differential Scanning Calorimetry (MTDSC), and to assess the differences between starch and cellulosic derivatives of methyl methacrylate and between two different drying methods used in their preparation.

METHODS

Graft copolymers of methyl methacrylate were synthesized and dried by oven or freeze-drying. Surface area measurements and different thermal analysis techniques (Differential Scanning Calorimetry (DSC), Thermogravimetric analysis (TGA) and MTDSC) were used to characterize these copolymers. Results. DSC was not sensitive enough to identify the T(g)s of the copolymers, however they were clearly identifiable by MTDSC. T(g) values obtained may depend on the method of preparation that also altered their physical characteristics e.g. specific surface area. Cellulose derivatives showed lower T(g)s than starch derivatives. The results also depended on the drying method used, thus, freeze dried products had slightly lower T(g)s than oven dried products.

CONCLUSIONS

MTDSC represents a useful thermal technique that allows the identification of glass transitions in these new copolymers with higher sensitivity and resolution than conventional DSC, separating the transition from overlapping phenomena such as decomposition or dehydration. The Tg of this new class of copolymers appeared to be dependent on polymer composition and drying method used.

Water distribution studies within cellulose ethers using differential scanning calorimetry. 1. Effect of polymer molecular weight and drug addition

Differential scanning calorimetry (DSC) was employed to characterize the distribution of water in gels produced from a series of hydroxypropylmethylcelluloses (HPMC, Methocel K-series) of different molecular weights (i.e., different viscosity grades). The presence of loosely bound water was characterized as pre-endothermic events occurring at temperatures below the main melting endotherm of free water. Both the magnitude and occurrence of these pre-endothermic events were affected by polymer molecular weight and gel storage time. In addition, the amount of water bound to the polymer depended on polymer molecular weight and gel storage time. The temperature at which frozen water melted within the gels was dependent on polymer concentration, with a depression of extrapolated endothermic melting peak onset occurring with an increase in polymer concentration. The addition of propranolol hydrochloride or diclofenac sodium, as model drugs, affected both the occurrence of pre-endothermic events and the distribution of water within the gels.

Release of propranolol hydrochloride from matrix tablets containing sodium carboxymethylcellulose and hydroxypropylmethylcellulose

Hydroxypropylmethylcellulose (HPMC) and three viscosity grades of sodium carboxymethylcellulose (NaCMC), namely NaCMC (Blanose 7H 4XF), NaCMC (Courlose P 800), and NaCMC (Courlose P 350), were investigated for their ability to provide a sustained release of propranolol hydrochloride from matrices. The rank order of release rate, in the absence of HPMC, was NaCMC (Blanose) < NaCMC P 800 < NaCMC P 350 for matrices containing 95-285 mg NaCMC, and was dependent on their viscosity grades. The effects of changing the ratio of HPMC to NaCMC (Blanose) and the drug/total polymer ratio were examined. The release rates decreased as the proportion of NaCMC in the matrices increased. Zero-order release of propranolol hydrochloride was obtained from matrices containing 285 mg 3:1 NaCMC (Blanose)/HPMC. Differential scanning calorimetry was used to quantify the moisture uptake by the polymers at 37 degrees C. Wafers containing NaCMC (Blanose) or 1:1 HPMC/NaCMC (Blanose) absorbed water similarly. A study of the erosion rates of matrices containing polymer only indicated that NaCMC (Blanose) eroded more quickly than HPMC. When propranolol hydrochloride was included in matrices containing NaCMC (Blanose), the erosion was reduced as a result of the insolubility of a complex formed between NaCMC and propranolol hydrochloride. The interaction between propranolol hydrochloride and NaCMC (Blanose) was confirmed by both dialysis and by monitoring the release of sodium ions from the matrices.

Water distribution studies within cellulose ethers using differential scanning calorimetry. 2. Effect of polymer substitution type and drug addition

The distribution of water within gels composed of a range of cellulose ether polymers of similar molecular weights (viscosity grades of 4000-6000 cP) but varying substitution types and levels was assessed by differential scanning calorimetry (DSC). Water loosely bound to the polymer was detected as one or more events appearing at the low-temperature side of the main endotherm for the melting of free water in DSC scans. Polymer substitution types and levels, and added drugs (50 mM propranolol hydrochloride or 50 mM diclofenac sodium) influenced the appearance of these melting events. Hydroxypropylcellulose (HPC) and hydroxypropylmethylcellulose (HPMC F4M) gels showed behavior different to that of the other polymers studied. It is thought that any water binding to HPC gels is tightly attached and is not visible as pre-endothermic events on DSC scans. The amount of water bound per polymer repeating unit (PRU) was influenced by and related to the degree of hydrophilic and hydrophobic substitution on the polymer backbone and by the inclusion of either drug. HPC gels had the highest bound water content after 96 h and this was probably related to the high percentage of hydrophilic hydroxypropoxyl substitutions in this polymer. In contrast, methylcellulose (MC A4M) had the lowest bound water content after 96 h storage, and this was explained by the lack of hydrophilic hydroxypropoxyl substitutions in the polymer.

Influence of drug:hydroxypropylmethylcellulose ratio, drug and polymer particle size and compression force on the release of diclofenac sodium from HPMC tablets

This study evaluates the relationship and influence of formulation and technological factors such as drug:hydroxypropylmethylcellulose (HPMC) ratio, particle size of the drug, particle size of HPMC and compression force, on drug release from matrices containing HPMC and diclofenac sodium as a model drug. The influence of these variables was assessed by multi-way analysis of variance. The results of the present study point out that the rate and mechanism of diclofenac sodium release from HPMC K15M matrices are mainly controlled by the drug:HPMC ratio. The drug and HPMC particle size also influence the drug release parameters, although to a lesser extent. Finally, the independence of the drug release from matrix tablets with respect to the compression force is reported.

Structure and behavior in hydrophilic matrix sustained release dosage forms: 4. Studies of water mobility and diffusion coefficients in the gel layer of HPMC tablets using NMR imaging

PURPOSE

The purpose of this study was to characterise the water mobility in the gel layer of hydrating HPMC tablets. Water mobility in the gel layer of different HPMCs was studied.

METHODS

NMR imaging, a non-invasive technique, has been used to measure the spatial distribution of self-diffusion coefficient (SDC) and T2 relaxation times across the gel layer.

RESULTS

It has been shown that there is a water mobility gradient across the gel layer of HPMC tablets. Although SDC and T2 relaxation times in the outer parts of the gel layer approached that of free water, in the inner parts they decreased progressively. Water mobility and SDC in the gel layer of different HPMCs appeared to vary with degree of substitution of the polymer and the lowest values were obtained across the gel layer of K4M tablets.

CONCLUSIONS

Water mobility varies across the gel layer of hydrating HPMC tablets and it is dependent on the degree of substitution of the polymer.

NMR microscopy of hydrating hydrophilic matrix pharmaceutical tablets

NMR microscopy has been used to monitor the formation of the gel layer in hydrating hydrophilic polymer tablets. Such tablets are used in the controlled delivery of drugs, where it has been found that the rate and extent of the swelling of the outer gel layer critically influences the kinetics of drug release. Tablets were hydrated in distilled water at 37 degrees C and then imaged at discrete time intervals using a 500 MHz microscope. The growth of the gel layer was clearly observed in time sequences of radial and axial sections. Axial images showed some interesting dimensional changes, with the gel at the flat surface of the tablet developing a concave shape. This is probably a reflection of the occurrence of uni-axial stress relaxation as hydration proceeds. Diffusion- and T2-weighted images provided evidence that the water in the gel layer is more strongly bound close to the dry core of the tablet than at the more fully hydrated outer surface. In images of tablets containing diclofenac, disruption of the gel layer was shown to occur primarily from the flat surfaces of the tablet, whilst the distribution of particles could be seen in tablets doped with insoluble calcium phosphate.