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

Thermal gelation of aqueous hydroxypropylmethylcellulose solutions with SDS and hydrophobic drug particles

The thermal gelation of hydroxypropylmethylcellulose (HPMC) solutions has been studied as a function of sodium dodecyl sulfate (SDS) concentration with and without griseofulvin, a model particulate BCS Class II drug by rheological measurements of gelation temperature (Tgel), steady-state viscosity (η) at 25 °C, and ζ-potential. Polymer adsorption on the drug was demonstrated by a decrease in η and potential in the absence of SDS. Griseofulvin had a synergistic effect on gelation which was attributed to an effective spanning of associated hydrophobic polymeric regions through interactions with the adsorbed polymer. Adding SDS offsets this effect on Tgel shielding hydrophobic interactions. Higher SDS concentrations had no effect on the particles surface as evidenced by constant ζ-potential and Tgel. Yet, polymeric chains are saturated and larger surfactant aggregates account for the increase in viscosity. Understanding the gelation mechanism and complex interactions of HPMC with surfactants and drugs is necessary for the design of pharmaceutical products and optimization of their performance properties.

Interplay between gelation and phase separation in aqueous solutions of methylcellulose and hydroxypropylmethylcellulose

Thermally induced gelation in aqueous solutions of methylcellulose (MC) and hydroxypropylmethylcellulose (HPMC) has been studied by rheological, optical microscopy, and turbidimetry measurements. The structural and mechanical properties of these hydrogels are dominated by the interplay between phase separation and gelation. In MC solutions, phase separation takes place almost simultaneously with gelation. An increase in the storage modulus is coupled to the appearance of a bicontinuous structure upon heating. However, a thermal gap exists between phase separation and gelation in the case of HPMC solutions. The storage modulus shows a dramatic decrease during phase separation and then rises in the subsequent gelation. A macroporous structure forms in the gels via "viscoelastic phase separation" linked to "double phase separation".

The effect of the physical states of binders on high-shear wet granulation and granule properties: a mechanistic approach towards understanding high-shear wet granulation process. Part I. Physical characterization of binders

In this study, the objective is to investigate the effect of the physical state of a binder on wet granulation and granule properties using a binary model system (CaCO(3)-binder), which is essential for understanding the mechanism of wet granulation when binder is added in a dry state. Part I focus on studying the phase behavior or the physical state change of four binders: PVP K12, K29/32, HPC, and HPMC, after exposure to either moisture or liquid water. Their interaction with water was studied by measuring the water sorption of binders and the binary blends of CaCO(3)-binder. Changes in the physical states of the binders at room temperature as a function of water content was monitored via dialysis experiments, and characterized by determining the glass transition temperatures (T(g)) of the binders with water. The results suggest that the PVP binders can absorb more water than the cellulosic binders which is same for binder alone and in the binary blends. PVP K12 undergoes a phase transition from the glassy state to the rubbery/solution state at much lower water content than PVP K29/32 (10% vs. 20%) at room temperature. The phase transition for HPC occurs with 10-15% water based on rheological measurements.

Extended-release oral drug delivery technologies: monolithic matrix systems

Oral drug delivery is the largest and the oldest segment of the total drug delivery market. It is the fastest growing and most preferred route for drug administration. Use of hydrophilic matrices for oral extended release of drugs is a common practice in the pharmaceutical industry. This chapter presents different polymer choices for fabrication of monolithic hydrophilic matrices and discusses formulation and manufacturing variables affecting the design and performance of the extended-release product by using selected practical examples.

New Cyclodextrin Hydrogels Cross-Linked with Diglycidylethers with a High Drug Loading and Controlled Release Ability

PURPOSE

The goal of the study is to develop new hydrogels based on cyclodextrins cross-linked with ethyleneglycol diglycidylether (EGDE) under mild conditions, to be used as carriers of amphiphilic drugs. Also, it aims to characterize the cross-linking and the drug loading and release processes.

METHODS

The cross-linking of hydroxypropyl-beta-cyclodextrin (HPbetaCD) with EGDE, in the absence or presence of hydroxypropylmethylcellulose (HPMC) Methocel K4M, was optimized applying oscillatory rheometry and Fourier transform infrared. Hydrogels were characterized regarding swelling in water, ability to load diclofenac, and release after different drying treatments.

RESULTS

Solutions of HPbetaCD (14.28%), without or with HPMC (0.2-1.0%), provided firm and transparent hydrogels after cross-linking with EGDE (14.28%), in which around two thirds of the OH groups were cross-linked. The incorporation of HPMC progressively reduced the gel time and the swelling degree of hydrogels. HPbetaCD hydrogels efficiently loaded diclofenac and sustained the release for several hours. The presence of HPMC slowed the release from swollen hydrogels, but promoted it from hydrogels dried before the loading and also before the release.

CONCLUSIONS

HPbetaCD hydrogels with good mechanical properties and tunable loading and release ability can be obtained by direct cross-linking with EGDE.

Effect of residual water content on the physico-chemical properties of sucralfate dried gel obtained by microwave drying

The purpose of this study was to investigate the physico-chemical characteristics of sucralfate humid gel dried by microwaves, in relation to the residual water content. Differential scanning calorimetry (DSC) allowed for the determination of the water state in sucralfate samples. Fourier-transform infrared (FT-IR) spectroscopy was used to monitor the changes in sucralfate gel structure induced by the microwave drying. A boundary value of total water content for sucralfate gel samples was found at 42% (w/w). Below this value only bound water was present, whereas above this value, the increase in total water was due to free water. In the physical form of gel, the strength of the coordination between sulfate anions and the positively charged aluminum hydroxide was dependent on the residual water content. The study of the sedimentation behavior of water suspensions prepared with dried sucralfate allowed for the evaluation of the retention of gel properties. We found that the microwave drying process affected the sedimentation of sucralfate dried gel suspensions independent of the residual water content: when suspensions were prepared from sucralfate dried gel powders containing more than 42% (w/w) of residual water, the sedimentation ratio was higher than 0.9. The non-gel powder suspension showed a sedimentation ratio of 0.68 +/- 0.02, whereas the sucralfate humid gel suspension did not sediment.

Water Distribution Studies Within Microcrystalline Cellulose and Chitosan using Differential Scanning Calorimetry and Dynamic Vapor Sorption Analysis

The objective of the study was to assess and compare the interaction and distribution of water within microcrystalline cellulose (MCC) and chitosan by differential scanning calorimetry (DSC) and dynamic vapor sorption analysis. The amounts of nonfreezing and freezing water in hydrated samples were determined from melting endotherms obtained by DSC. After accounting for the percent crystallinity of MCC and chitosan, no statistically significant difference was observed in their ability to bind water molecules per repeating unit at the minimum water content at which freezing water is evident. Exposure of chitosan to water for 30 min was sufficient to achieve equilibration at 61% w/w actual water content. The moisture sorption profiles were analyzed according to the GAB and Young and Nelson equations. The adsorbed monolayer, externally adsorbed moisture, and internally absorbed moisture were not statistically different for MCC and chitosan after accounting for the amorphous content of the polymers. These studies suggest that chitosan can act as a "molecular sponge," and thus aid in the production of beads by extrusion and spheronization.

Studies on the interaction of water with ethylcellulose: effect of polymer particle size

The purpose of this research was to investigate the interaction of water with ethylcellulose samples and assess the effect of particle size on the interaction. The distribution of water within coarse particle ethylcellulose (CPEC; average particle size 310 micro m) and fine particle ethylcellulose (FPEC; average particle size 9.7 micro m) of 7 cps viscosity grade was assessed by differential scanning calorimetry (DSC) and dynamic vapor sorption analysis. The amounts of nonfreezing and freezing water in hydrated samples were determined from melting endotherms obtained by DSC. An increase in water content resulted in an increase in the enthalpy of fusion of water for the two particle size fractions of EC. The amount of nonfreezable water was not affected by the change in particle size at low water contents. Exposure of ethylcellulose to water for 30 minutes is sufficient to achieve equilibration within the hydrated polymer at 47% wt/wt water content. The moisture sorption profiles were analyzed according to the Guggenheim-Anderson-de Boer (GAB) and Young and Nelson equations, which can help to distinguish moisture distribution in different physical forms. The amount of externally adsorbed moisture was greater in the case of FPEC. Internally absorbed moisture was evident only with the CPEC. In light of these results, an explanation is offered for the success of FPEC in wet-granulation methods where CPEC was not successful.

Thermal gelation of aqueous curdlan suspension: preparation of curdlan jelly

Curdlan jelly was prepared by heating an aqueous curdlan suspension at 70 degrees C for 5 min. Theophylline, as a model drug, was entrapped in the jelly network. Curdlan jelly had a hardness comparable with that of commercially available jelly for confectionary. Syneresis was observed for 8 days after the preparation and was not detected during the experimental term from the gel prepared from 10% w/v curdlan suspension. Release of theophylline from the jellies was sustained, and was increasingly delayed with an increase in the curdlan concentration. An aqueous curdlan suspension was studied by means of differential scanning calorimetry (DSC) up to 80 degrees C and down to -25 degrees C, and subsequent re-heating to 30 degrees C. Enthalpy increased with the concentration of curdlan, while the temperature at the onset of the endothermic peak decreased with the concentration of curdlan. The enthalpy due to thermal gelation of 1 mg curdlan was 12.2 mJ. An increase in curdlan concentration decreased the enthalpy and lowered the onset temperature of the endothermic peak during the DSC re-heating scan. The results are due to an increase in the amount of non-freezing water and freezing bound water and a decrease in free water. The number of water molecules entrapped in the curdlan jelly as non-freezing water was 8.1 per glucopyranose residue.

Effect of polymer hydration on the kinetic release of drugs: a study of ibuprofen and ketoprofen in HPMC matrices

Samples of drug/hydroxypropylmethylcellulose (HPMC) mixtures and matrices (drug/HPMC mixtures plus excipients) were allowed to equilibrate in closed chambers with defined relative humidities (RHs). Their water uptake and drug release were evaluated by differential scanning calorimetry/thermogravimetric analysis and dissolution studies, respectively. Analysis of the thermal behaviors of the drug/HPMC mixtures and of the polymer alone, as functions of RH, leads to the conclusion that most of the hydration water is retained by the polymer, and points to the occurrence of different types of hydration water, from the strongly polymer-bound water molecules at RH values up to 81%, to the almost "free water" for RH values close to 100%. In addition, application of the Korsmeyer model to the dissolution results leads to the conclusion that the rate determining dissolution processes are predominantly of the fickian type.

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.